I have used Kubuntu on my Dell XPS M1330 laptop (NVIDIA GeForce 8400M) since 2008 with no problems.  Immediately after upgrading from Oneiric to Precise I began having problems with X freezing at 100% CPU while playing video, usually with Firefox playing Flash in YouTube.  The screen freezes completely, and I can't switch to other VTs.  I can SSH in and see that X is at 100% CPU.  Killing VLC, Firefox, and plugin-container doesn't unfreeze the system or stop X from using 100% CPU.  Looking in dmesg I see:

[ 2140.551099] NVRM: GPU at 0000:01:00.0 has fallen off the bus.
[ 2140.551107] NVRM: GPU at 0000:01:00.0 has fallen off the bus.
[ 2140.551133] NVRM: os_pci_init_handle: invalid context!
[ 2140.551139] NVRM: os_pci_init_handle: invalid context!
[ 2140.551206] NVRM: os_pci_init_handle: invalid context!
[ 2140.551224] NVRM: os_pci_init_handle: invalid context!
[ 2140.551229] NVRM: os_pci_init_handle: invalid context!
[ 2142.079045] irq 16: nobody cared (try booting with the "irqpoll" option)
[ 2142.079051] Pid: 0, comm: BFS/0 Tainted: P         C O 3.3.6-pf-adp+ #6
[ 2142.079054] Call Trace:
[ 2142.079061]  [<c154ddf6>] ? printk+0x2d/0x2f
[ 2142.079067]  [<c10b4139>] __report_bad_irq+0x29/0xd0
[ 2142.079070]  [<c10b43ae>] note_interrupt+0x11e/0x1d0
[ 2142.079174]  [<f9bad083>] ? nv_kern_isr+0x33/0x70 [nvidia]
[ 2142.079178]  [<c10b21ee>] handle_irq_event_percpu+0x9e/0x200
[ 2142.079182]  [<c10b4cf0>] ? handle_fasteoi_irq+0xd0/0xd0
[ 2142.079186]  [<c15569e0>] ? nmi_stack_correct+0x2f/0x34
[ 2142.079190]  [<c10269e8>] ? default_spin_lock_flags+0x8/0x10
[ 2142.079194]  [<c101df4d>] ? __io_apic_modify_irq+0x7d/0x90
[ 2142.079198]  [<c10b238b>] handle_irq_event+0x3b/0x60
[ 2142.079201]  [<c10b4c20>] ? unmask_irq+0x30/0x30
[ 2142.079204]  [<c10b4c6e>] handle_fasteoi_irq+0x4e/0xd0
[ 2142.079206]  <IRQ>  [<c155d3b2>] ? do_IRQ+0x42/0xc0
[ 2142.079213]  [<c1008638>] ? sched_clock+0x8/0x10
[ 2142.079217]  [<c1060ffb>] ? sched_clock_local+0xcb/0x1c0
[ 2142.079221]  [<c155d2f0>] ? common_interrupt+0x30/0x38
[ 2142.079225]  [<c10600d8>] ? build_sched_domains+0x168/0x7d0
[ 2142.079230]  [<c1315fef>] ? arch_local_irq_enable+0x5/0xb
[ 2142.079233]  [<c13169cb>] ? acpi_idle_enter_simple+0xf3/0x133
[ 2142.079237]  [<c1448d9d>] ? cpuidle_idle_call+0xad/0x250
[ 2142.079241]  [<c100174c>] ? cpu_idle+0x9c/0xe0
[ 2142.079244]  [<c1531825>] ? rest_init+0x5d/0x68
[ 2142.079249]  [<c17f5745>] ? start_kernel+0x357/0x35d
[ 2142.079252]  [<c17f517f>] ? loglevel+0x2b/0x2b
[ 2142.079255]  [<c17f5078>] ? i386_start_kernel+0x78/0x7d
[ 2142.079257] handlers:
[ 2142.079340] [<f9bad050>] nv_kern_isr
[ 2142.079342] Disabling IRQ #16

I've never, ever had this problem until "upgrading" to Oneiric.  Now I have this crash often, at least six times since upgrading.  It may be a few days, a few hours, or a few minutes between crashes.  It's happened twice today.  Sometimes I can SAK+K twice to kill X and restart KDM, but other times the SysRq keys do nothing and I have to reset the system.  So far, when it's happened, it's always been while playing video with VLC with Firefox and Flash running.

I'm currently on 295.40.  I will try upgrading to 295.49 and see if it resolves the crashes.  I rarely use any 3D stuff on my laptop, so I doubt I can comment on performance issues.