I have the similar problem too. The system freezes, keyboard and mouse don't respond.
I don't use encrypted fs, don't know whether ktorrent tries to rescan downloaded data.
But I get some messages in /var/log/messages. Something like this:

kernel: [176880.392142] ktorrent      D f38bbb5c     0 10352      1 0x00000000
kernel: [176880.392153]  f38bbb6c 00000086 00000002 f38bbb5c 00011200 c05d8a20 c08c3700 c08c3700
kernel: [176880.392167]  c3c9e1b6 0000a0c0 c08c3700 c08c3700 c3c62877 0000a0c0 00000000 c08c3700
kernel: [176880.392180]  c08c3700 ea198000 00000001 ea198000 c2921700 f38bbbb8 f38bbb7c c05c69b1
kernel: [176880.392193] Call Trace:
kernel: [176880.392209]  [<c05c69b1>] io_schedule+0x61/0xa0
kernel: [176880.392219]  [<c023d668>] sync_buffer+0x38/0x40
kernel: [176880.392226]  [<c05c717d>] __wait_on_bit+0x4d/0x70
kernel: [176880.392233]  [<c023d630>] ? sync_buffer+0x0/0x40
kernel: [176880.392240]  [<c023d630>] ? sync_buffer+0x0/0x40
kernel: [176880.392248]  [<c05c724b>] out_of_line_wait_on_bit+0xab/0xc0
kernel: [176880.392257]  [<c0165e60>] ? wake_bit_function+0x0/0x50
kernel: [176880.392264]  [<c023d62e>] __wait_on_buffer+0x2e/0x30
kernel: [176880.392271]  [<c023e602>] __bread+0x72/0xa0
kernel: [176880.392279]  [<c0277d02>] ext3_get_branch+0x72/0xf0
kernel: [176880.392287]  [<c0279763>] ext3_get_blocks_handle+0x83/0x4a0
kernel: [176880.392297]  [<c033f842>] ? submit_bio+0x72/0x100
kernel: [176880.392304]  [<c0279c30>] ext3_get_block+0xb0/0x100
kernel: [176880.392313]  [<c0245af6>] do_mpage_readpage+0x1c6/0x730
kernel: [176880.392322]  [<c01e2cd9>] ? ____pagevec_lru_add+0x179/0x190
kernel: [176880.392329]  [<c01e2d37>] ? __lru_cache_add+0x47/0x60
kernel: [176880.392337]  [<c01da07c>] ? add_to_page_cache_lru+0x5c/0x70
kernel: [176880.392345]  [<c0246197>] mpage_readpages+0xc7/0x100
kernel: [176880.392352]  [<c0279b80>] ? ext3_get_block+0x0/0x100
kernel: [176880.392361]  [<c01dfcc9>] ? __alloc_pages_nodemask+0xd9/0x1c0
kernel: [176880.392368]  [<c0278590>] ? ext3_readpages+0x0/0x20
kernel: [176880.392375]  [<c02785ae>] ext3_readpages+0x1e/0x20
kernel: [176880.392382]  [<c0279b80>] ? ext3_get_block+0x0/0x100
kernel: [176880.392390]  [<c01e1f6c>] __do_page_cache_readahead+0x14c/0x210
kernel: [176880.392398]  [<c01e2056>] ra_submit+0x26/0x30
kernel: [176880.392406]  [<c01db75f>] filemap_fault+0x3cf/0x400
kernel: [176880.392413]  [<c035a1d9>] ? copy_to_user+0x39/0x130
kernel: [176880.392422]  [<c01f25a0>] __do_fault+0x40/0x570
kernel: [176880.392429]  [<c01f4566>] handle_mm_fault+0x146/0x400
kernel: [176880.392438]  [<c0546a40>] ? udp_ioctl+0x0/0x80
kernel: [176880.392447]  [<c05cbed6>] do_page_fault+0x146/0x440
kernel: [176880.392456]  [<c04e7d35>] ? sys_socketcall+0x175/0x2a0
kernel: [176880.392464]  [<c05cbd90>] ? do_page_fault+0x0/0x440
kernel: [176880.392471]  [<c05c9307>] error_code+0x73/0x78
kernel: [176880.392479]  [<c05c0000>] ? identify_cpu+0xc4/0x229

It appears many times, every time starting with "kernel: [...] ktorrent D f38bbb5c 0 10352 1 0x00000000", so it is somehow related to ktorrent.

Sometimes between this lines the info about memory usage is shown:
kernel: [177192.740206] Mem-Info:
kernel: [177192.740210] DMA per-cpu:
kernel: [177192.740215] CPU    0: hi:    0, btch:   1 usd:   0
kernel: [177192.740221] CPU    1: hi:    0, btch:   1 usd:   0
kernel: [177192.740225] Normal per-cpu:
kernel: [177192.740230] CPU    0: hi:  186, btch:  31 usd: 178
kernel: [177192.740235] CPU    1: hi:  186, btch:  31 usd: 143
kernel: [177192.740239] HighMem per-cpu:
kernel: [177192.740244] CPU    0: hi:  186, btch:  31 usd: 135
kernel: [177192.740249] CPU    1: hi:  186, btch:  31 usd: 117
kernel: [177192.740260] active_anon:298847 inactive_anon:156151 isolated_anon:2336
kernel: [177192.740263]  active_file:356 inactive_file:737 isolated_file:0
kernel: [177192.740266]  unevictable:0 dirty:2 writeback:237 unstable:0
kernel: [177192.740269]  free:11818 slab_reclaimable:5084 slab_unreclaimable:19326
kernel: [177192.740272]  mapped:5928 shmem:25602 pagetables:4420 bounce:0
kernel: [177192.740287] DMA free:6612kB min:64kB low:80kB high:96kB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:15804kB mlocked:0kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:136kB slab_unreclaimable:104kB kernel_stack:16kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? yes
kernel: [177192.740299] lowmem_reserve[]: 0 865 2006 2006
kernel: [177192.740319] Normal free:40204kB min:3728kB low:4660kB high:5592kB active_anon:338836kB inactive_anon:339044kB active_file:612kB inactive_file:1044kB unevictable:0kB isolated(anon):768kB isolated(file):0kB present:885944kB mlocked:0kB dirty:0kB writeback:212kB mapped:8760kB shmem:46044kB slab_reclaimable:20200kB slab_unreclaimable:77200kB kernel_stack:3824kB pagetables:4612kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:224 all_unreclaimable? no
kernel: [177192.740333] lowmem_reserve[]: 0 0 9134 9134
kernel: [177192.740351] HighMem free:456kB min:512kB low:1740kB high:2972kB active_anon:856552kB inactive_anon:285560kB active_file:812kB inactive_file:1904kB unevictable:0kB isolated(anon):8576kB isolated(file):0kB present:1169168kB mlocked:0kB dirty:8kB writeback:736kB mapped:14952kB shmem:56364kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:13068kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:352 all_unreclaimable? no
kernel: [177192.740365] lowmem_reserve[]: 0 0 0 0
kernel: [177192.740372] DMA: 7*4kB 21*8kB 17*16kB 14*32kB 11*64kB 11*128kB 6*256kB 2*512kB 1*1024kB 0*2048kB 0*4096kB = 6612kB
kernel: [177192.740393] Normal: 9299*4kB 6*8kB 1*16kB 6*32kB 5*64kB 1*128kB 1*256kB 0*512kB 0*1024kB 1*2048kB 0*4096kB = 40204kB
kernel: [177192.740414] HighMem: 22*4kB 4*8kB 11*16kB 5*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 456kB
kernel: [177192.740435] 36030 total pagecache pages
kernel: [177192.740439] 9304 pages in swap cache
kernel: [177192.740444] Swap cache stats: add 770325, delete 761021, find 698606/719042
kernel: [177192.740449] Free swap  = 0kB
kernel: [177192.740452] Total swap = 2008120kB
kernel: [177192.753276] 521904 pages RAM
kernel: [177192.753282] 294594 pages HighMem
kernel: [177192.753286] 9235 pages reserved
kernel: [177192.753289] 21326 pages shared
kernel: [177192.753293] 486620 pages non-shared

Here I can see "Free swap = 0kB". After some time, kernel frees the memory and Xserver restarts.