Benchmarks dropping when running 30 minutes

[stargrasske]

hi,

Using Passmark Performance Test I noticed the following:
- When I put off the power and wait for an hour or so CPU benchmarks seem normal or close to normal...
- When after 30 minutes or so I do the entire test again CPU benchmarks and 2D-3D graphic benchmarks have dropped (a lot)
- After normal restart CPU benchmarks go up a little, but not as much as when it comes from total cold state

AMD Athlon 64 X2 6400+
Asus M2N-MX SE PLUS
Transcend 1024 Mb - 800 Mhz
Msi NX8500GT
XP SP3

While rebooting and checking Bios it seemed to struggle too...

Already tried different graphics card, didn't seem to make much of a difference... tried replacing RAM too... doesn't seem to be the problem...

Asus PC Probe II shows the motherboard at temperatures of 45-48°C
CPU goes up to 50°C

Doesn't seem to be a software prob, 'cause I reinstalled couple of times now. Doesn't change... drivers 've been up- and downgraded... didn't help either.

Could this be a heat problem or is it more likely a malfunction of CPU/motherboard ?