CPU -30 +39 -120 +120 Degrees

Guys/Girls

Ive got a HP Pavilion A1550 (not my machine)
its a Pentium D 2.66ghz just replaced the mainboard
now I've got 2 mainboards where the temp is going up & down completly crazy..
the machine seems to run stable but the fan is spinning up to 4000rpm then stoping then going full ball again.
i've reapplied thermal compound several times..

Does anyone have any idea what the problem would be..
I have the slightest inkling that it could be the power supply.
Anyone have any ideas.

Cheers
Mr sgtRedneck