Computer turns itself off with a siren!

[pickers]

Tech specs:
EpoX 8K9A7i with an athlon xp2800+ (barton I do believe)
2x 256 ram sticks
enermax 433 PSU (coupla years old).

About 2 months ago I realised CPU was running slow, and so flashed the bios with newest one, and it now recognised the CPU and its true speed... great! But since:
At random times (Doom3, once in farcry and once in richard burns rally) the computer would turn off just like a power cut, and the internal speak would go off with a two tone slow siren for about 30 seconds, and power on button did nothing untill a couple of mins later.
CPU would get to 52 ish under load. Tried a new case (antec P160), and new PSU, but the PSU ran quite hot, so I put my old enermax back in. Now get cooler case temp, and a full load temp touching 50. Still- it seemed to work ok (for 3-4 weeks).

Now this week Its happened twice, once when I was running winamp and writing a heafty lab report in word, and again when running winamp and playing a rendering java simulation in IE. Seems to happen under load... so
bad RAM?
bad CPU?
Temp probs?
Bad voltage?

Help!!! :/

[Gizmo]

The two-tone siren is generally an overtemp alarm. This may be implemented with hardware or in software or both (depends on the mobo, and I'm not familiar with yours).

[cloasters]

Welcome to AOA Forums, pickers!
Some mainboards lie to you about your processor temperature in BIOS. Some Asus mobos are famous for this. My P4P800D says 30C when the CPU temp is really 40C or 42C.

[pickers]

well the manual says that the computer will shut itself off if the cpu reaches ~ 110 degrees. surely its not getting that high when MBM is reading 50ish?

[Kaitain]

Check your BIOS setting for overtemp shut down - most motherboards offer a variety of optional shut down temperatures ranging from the paranoid (60ºC) to the brave (95ºC). If you've left it set to 60ºC, are using air cooling, and have a bit of dust in the case then the computer's just doing its job

[pickers]

no no - its off by default, and still is