Hi to all, 

same problems here:

Relevant hardware:
Asus A8N SLi Deluxe;
2x 250GB 7200 rpm Western Digital Hard drives, SATA-3
1x DVD-RW, NEC 3500 A
1x Compaq DVD-ROM (slave on ATA cable)

Symptoms:
copying a large file (700MB) to the same hard drive reads around 8 to 13 MBps at most; very occasionally jumps to 20MBps.
Same sort of speed copying to the other hard drive, formatted in NTFS (yes, for the Win XP install that I would like to dump, if only Ubuntu was more reliable.)

uname -a:
Linux Le-Grand-bleu 2.6.24-19-generic #1 SMP Fri Jul 11 23:41:49 UTC 2008 i686 GNU/Linux

cubytus@Le-Grand-bleu:~$ dmesg | grep ata
[    0.000000]  BIOS-e820: 000000003fff3000 - 0000000040000000 (ACPI data)
[   71.301111] Memory: 1027360k/1048512k available (2177k kernel code, 20460k reserved, 1006k data, 368k init, 131008k highmem)
[   71.301125]       .data : 0xc0320474 - 0xc041bdc4   (1006 kB)
[   75.481984] libata version 3.00 loaded.
[   76.261838] pata_amd 0000:00:06.0: version 0.3.10
[   76.262715] scsi0 : pata_amd
[   76.262906] scsi1 : pata_amd
[   76.263445] ata1: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0xf000 irq 14
[   76.263447] ata2: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0xf008 irq 15
[   76.909093] ata2.00: ATAPI: _NEC DVD_RW ND-3500AG, 2.1B, max UDMA/33
[   76.909112] ata2.01: ATAPI: COMPAQ  DVD-ROM GD-8000, 0011, max UDMA/66
[   76.909123] ata2.01: limited to UDMA/33 due to 40-wire cable
[   77.072902] ata2.00: configured for UDMA/33
[   77.244770] ata2.01: configured for UDMA/33
[   77.251491] sata_nv 0000:00:07.0: version 3.5
[   77.251868] sata_nv 0000:00:07.0: Using ADMA mode
[   77.253359] scsi2 : sata_nv
[   77.253545] scsi3 : sata_nv
[   77.253695] ata3: SATA max UDMA/133 cmd 0x9f0 ctl 0xbf0 bmdma 0xd800 irq 19
[   77.253698] ata4: SATA max UDMA/133 cmd 0x970 ctl 0xb70 bmdma 0xd808 irq 19
[   77.564337] ata3: SATA link down (SStatus 0 SControl 300)
[   77.876091] ata4: SATA link down (SStatus 0 SControl 300)
[   77.876499] sata_nv 0000:00:08.0: Using ADMA mode
[   77.876618] scsi5 : sata_nv
[   77.876842] scsi6 : sata_nv
[   77.876968] ata5: SATA max UDMA/133 cmd 0x9e0 ctl 0xbe0 bmdma 0xc400 irq 16
[   77.876970] ata6: SATA max UDMA/133 cmd 0x960 ctl 0xb60 bmdma 0xc408 irq 16
[   78.343727] ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[   78.351865] ata5.00: ATA-7: WDC WD2500KS-00MJB0, 02.01C03, max UDMA/133
[   78.351868] ata5.00: 488397168 sectors, multi 1: LBA48 
[   78.359842] ata5.00: configured for UDMA/133
[   78.827344] ata6: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[   78.835466] ata6.00: ATA-7: WDC WD2500KS-00MJB0, 02.01C03, max UDMA/133
[   78.835468] ata6.00: 488397168 sectors, multi 1: LBA48 
[   78.843471] ata6.00: configured for UDMA/133
[   78.843566] ata5: bounce limit 0xFFFFFFFFFFFFFFFF, segment boundary 0xFFFFFFFF, hw segs 61
[   78.843911] ata6: bounce limit 0xFFFFFFFFFFFFFFFF, segment boundary 0xFFFFFFFF, hw segs 61
[   78.844643] sata_sil 0000:05:0a.0: version 2.3
[   78.844876] sata_sil 0000:05:0a.0: Applying R_ERR on DMA activate FIS errata fix
[   78.845861] scsi7 : sata_sil
[   78.845997] scsi8 : sata_sil
[   78.846135] scsi9 : sata_sil
[   78.846269] scsi10 : sata_sil
[   78.846292] ata7: SATA max UDMA/100 mmio m1024@0xd400d000 tf 0xd400d080 irq 20
[   78.846295] ata8: SATA max UDMA/100 mmio m1024@0xd400d000 tf 0xd400d0c0 irq 20
[   78.846298] ata9: SATA max UDMA/100 mmio m1024@0xd400d000 tf 0xd400d280 irq 20
[   78.846300] ata10: SATA max UDMA/100 mmio m1024@0xd400d000 tf 0xd400d2c0 irq 20
[   79.155085] ata7: SATA link down (SStatus 0 SControl 310)
[   79.466839] ata8: SATA link down (SStatus 0 SControl 310)
[   79.778591] ata9: SATA link down (SStatus 0 SControl 310)
[   80.090347] ata10: SATA link down (SStatus 0 SControl 310)
[   82.040465] EXT3-fs: mounted filesystem with ordered data mode.
[   82.789945] scsi 4:0:0:0: Direct-Access     Kingston DataTraveler 2.0 1.00 PQ: 0 ANSI: 2
[  144.695715] EXT3-fs: mounted filesystem with ordered data mode.

Don't have results for a previous version of Ubuntu since it's my first install in this computer.

I get similar speeds on my father's computers, which sports a different M/B and a 160GB ATA/100 hard drive (7 year-old computer, as opposed to mine which is 3)

Here is a sample of the tested speeds in hdparm: (still slow for a supposed 3Gbps STA link, but better than IRL)
/dev/sda:
 Timing cached reads:   1326 MB in  2.00 seconds = 663.08 MB/sec
 Timing buffered disk reads:  174 MB in  3.01 seconds =  57.76 MB/sec

Tell me how I can be of further help on this bug since I have access to two different computers with similar problem.

Cheers,
Cubytus