Latitude D600 temperature oddity

I have an intermittent problem: My D600 will show a steady processor temp of 85.5C(per Mobile Meter). When this happens, the processor will not step up above 600mhz and is thus very sluggish. I don't think the temp reading is accurate, because it will sometimes kick back down to a normal range in the high 30s or low 40s and run normally for a while. Is this indicative of a temp sensor problem or something like that? Thanks.