Very Odd Heating Problem

[TiTaNs_ReVeNgE]

So I've been running with my stock AMD64 cooler for over 2 years now. I bought an A64 4000+ not too long ago and tried overclocking it. Got a decent 10% more but it was heating up too much, around 57C (135F) at full load. So I bought an Arctic Cooling Freezer 64 Pro, installed it, and suddenly I'm getting 90C at full load. It must be wrong, has to be the thermometer. I then run Prime 95 and get errors within 20 seconds. So I put the stock cooler back in ready to write a bad review on the new cooler, when I notice I'm still running 54C idle (yes, idle). Which doesn't make sense since I was running 40C idle on it this morning. I figure the thermometer still isn't reading correctly so I try a Prime 95 run again, and like with the Freezer, I'm getting around 90C at full load and errors again just as quick.

Anyone have any idea what could have went wrong?