Homebuilt PC crashing randomly

[PriitM]

I got 2.5 year old system here what crashes randomly, it can happen at night when torrent client is working(then monitor stays in stand-by mode although PC is working), it can happen while playing StarCraft(1998 year game), it can happen while browsing the web. Crash turns PC completely unresponsive, keyboard and mouse won't work(not even num lock and such keys), audio will stop playing.

I have noticed that sometimes booting OS will get stuck at 'loading kernel modules...' and sometimes PC speaker won't beep.

Log files have no entry about the crash.

Issues started happening around week ago. No hardware chances since then.

Temperatures are relatively high, but nothing over the limits and they've been like this for a while.

So far I have:
Clean installed operating systems - Debian Testing, XP Pro
Done memtest - all good
Done seatools test - all good
Replaced HDD - no change
Replaced PSU - no change

Motherboard and graphics adapter are fairly new if that matters, maybe 3 months at most.

AMD X2 4200+ Toledo s939
Asus A8NE-FM(OEM for FuSi), nForce4
2x 1GB A-Data PC3200 CL3
2x 1GB Corsair XMS3200 CL2
Seagate ST3250620AS
MSI Geforce 7300LE
Canyon CN-WF511 RT61 PCI WiFi adapter
Sound Blaster Audigy SE
Card reader
DVD-RW
400W PSU

 

[PriitM]

For some reason I can't edit my post...
Update:
Booted PC with only essential parts: mobo, cpu, cooling, psu, hdd, 1 stick of ram, video card - still crashing. But I noticed one weird thing - CPU fan was changing speed