weird, did you try removing the fpu?  Maybe some pins are crossed or corroded.  I know from experience I cut fpus off of bad blow up boards to put in color classic and LC's and if those pins are not striaght it can get crazy.  Just a thought.  Also I could not read the speed on the fpu, if its 16mhz and the board is 25mhz or better that may be the heat issue and a heat sink on it may help.  

Its not been upped to 33 mhz has it? if so that could be overheating the cpu also and it would need a heat sink.

Just suggestions to look at.  Did not see any of this mentioned.