Some things to try:
Make sure the A3640 is seated correctly. Check the CPU clock jumpers. IIRC, (and I could be wrong), for the 3640 they have to be set to int.
Also check things like RAM being seated properly. The A4000 has really, really crappy SIMM sockets; the worst i've ever seen infact. My A4000 has a wad of paper jammed between the mediator busboard and the SIMMs to hold them in place!
Also check by the battery for signs of corrosion on the tracks. Also, there are a couple of resistors by the Paula that are known to leak (causing the loss of paula audio; its a very common fault on A4000s, don't know which revisions this applies to though. Unfortunatly the paula audio on mine is dead through this).
