At random times (usually when watching a movie in XBMC), the computer locks up. I can still sometimes SSH in and get the 'dmesg' output before that locks up too. A hard reboot is usually required to get things going again.
I have cut out the date/time/server columns for easier reading, please do ask if these seem relevant omissions...

System:
Ubuntu 11.04 (2.6.38-8-server) x64
X11 installed with IceWm (and XBMC)
Core 2 Duo E8400 @ 3.00GHz
8 GB RAM
Asus P5Q premium motherboard
Primary harddrive: OCZ Vertex 2 60 GB (SSD)
Other harddrives: various 750GB, 1TB, 1.5TB & 2TB (WD & samsung)

Any important information I am not supplying is purely a sign of my incompetence in these matters, so please do ask and excuse me for my inabilities...

 invalid opcode: 0000 [#1] SMP

 last sysfs file: /sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_map

 CPU 0

 Modules linked in: parport_pc ppdev vesafb snd_hda_codec_analog tuner_simple tuner_types wm8775 tda9887 tda8290 tea5767 tuner cx25840 ir_lirc_codec lirc_dev snd_hda_intel snd_hda_codec snd_hwdep snd_pcm ir_sony_decoder snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq rc_rc6_mce ivtv ir_jvc_decoder cx2341x i2c_algo_bit v4l2_common mceusb videodev ir_rc6_decoder ir_rc5_decoder snd_timer ir_nec_decoder nvidia(P) btusb bluetooth rc_core v4l2_compat_ioctl32 tveeprom snd_seq_device pata_marvell psmouse shpchp serio_raw snd asus_atk0110 soundcore snd_page_alloc lp parport firewire_ohci firewire_core crc_itu_t r8169 sky2 ahci libahci



 Pid: 4597, comm: xbmc.bin Tainted: P            2.6.38-8-server #42-Ubuntu System manufacturer P5Q Premium/P5Q Premium

 RIP: 0010:[<ffffffff8119bc4a>]  [<ffffffff8119bc4a>] do_mpage_readpage+0x9a/0x510

 RSP: 0018:ffff88021f5a59d8  EFLAGS: 00210246

 RAX: 0000000000000020 RBX: ffff88021f5a5ac8 RCX: 0000000000000000

 RDX: 0000000000000001 RSI: 0000000015e36fe0 RDI: 0000000000000000

 RBP: ffff88021f5a5a98 R08: ffff88021f5a5ac8 R09: ffff88021f5a5b38

 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000

 R13: 000000000000b148 R14: 0000000000000001 R15: ffff8802067034b8

 FS:  00007f3f34eb1700(0000) GS:ffff8800cfc00000(0000) knlGS:0000000000000000

 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033

 CR2: 00007feb8d515000 CR3: 000000021d744000 CR4: 00000000000406f0

 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000

 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400

 Process xbmc.bin (pid: 4597, threadinfo ffff88021f5a4000, task ffff88021f63db80)

 Stack:

  ffff88021f5a5a28 ffffffff8116019d ffff88021f5a5b40 0000002000000003

  ffff88021f5a5b38 ffff880206703370 ffffea0006d800f8 0000000000000000

  ffffea0006d800f8 0000000c811270b5 ffff88021f5a5a68 ffffffff8110bdba

 Call Trace:

  [<ffffffff8116019d>] ? mem_cgroup_cache_charge+0xed/0x130

  [<ffffffff8110bdba>] ? add_to_page_cache_locked+0xea/0x160

  [<ffffffff8119c232>] mpage_readpages+0x102/0x150

  [<ffffffff812063e0>] ? ext4_get_block+0x0/0x20

  [<ffffffff812063e0>] ? ext4_get_block+0x0/0x20

  [<ffffffff81149475>] ? alloc_pages_current+0xa5/0x110

  [<ffffffff8120157d>] ext4_readpages+0x1d/0x20

  [<ffffffff81116a9b>] __do_page_cache_readahead+0x14b/0x220

  [<ffffffff81116ed1>] ra_submit+0x21/0x30

  [<ffffffff81116ff5>] ondemand_readahead+0x115/0x230

  [<ffffffff811171a0>] page_cache_async_readahead+0x90/0xc0

  [<ffffffff8110b184>] ? file_read_actor+0xd4/0x170

  [<ffffffff812de72e>] ? radix_tree_lookup_slot+0xe/0x10

  [<ffffffff8110c521>] do_generic_file_read.clone.23+0x271/0x450

  [<ffffffff8110d1ba>] generic_file_aio_read+0x1ca/0x240

  [<ffffffff8100a82e>] ? __switch_to+0x20e/0x2f0

  [<ffffffff81164c82>] do_sync_read+0xd2/0x110

  [<ffffffff8108b61c>] ? hrtimer_try_to_cancel+0x4c/0xe0

  [<ffffffff81279083>] ? security_file_permission+0x93/0xb0

  [<ffffffff81164fa1>] ? rw_verify_area+0x61/0xf0

  [<ffffffff81165463>] vfs_read+0xc3/0x180

  [<ffffffff81165571>] sys_read+0x51/0x90

  [<ffffffff8100bfc2>] system_call_fastpath+0x16/0x1b

 Code: ff ff 48 c7 85 78 ff ff ff 00 00 00 00 49 d3 ee b9 0c 00 00 00 2b 4d 8c 48 8b b2 c8 00 00 00 ba 01 00 00 00 41 0f af c6 49 d3 e5 <0f> 36 4d 8c 4c 01 e8 d3 e2 4c 8d 44 16 ff 48 8b 53 20 49 d3 f8

 RIP  [<ffffffff8119bc4a>] do_mpage_readpage+0x9a/0x510

  RSP <ffff88021f5a59d8>

 ---[ end trace ac6cd2f4692205a3 ]---

Please note that the error is ALWAYS occuring at do_mpage_readpage+0x9a/0x510 with the same numbers after it. I've tried to come up with the possible meaning of these, but couldn't get any further.

I've also noticed that the top block from the call trace is always the following with the exact same numbers:

  [<ffffffff8116019d>] ? mem_cgroup_cache_charge+0xed/0x130

  [<ffffffff8110bdba>] ? add_to_page_cache_locked+0xea/0x160

  [<ffffffff8119c232>] mpage_readpages+0x102/0x150

  [<ffffffff812063e0>] ? ext4_get_block+0x0/0x20

  [<ffffffff812063e0>] ? ext4_get_block+0x0/0x20

Could this indicate a hard drive issue, a RAM issue or something else entirely?