Could be quite a number of things. The green screen indicates a failed chipmem test, but things not related to the chipmem can cause that test to fail.
But for a start, simple things you could try is to clean the chip mem SIMM slot with isopropyl alchohol and an old toothbrush. The same for the edge connector on the memory module.
Check for bad solder coints on the RAM address decoder (bridgette) or just try replacing it.
Fit a known good CPU board, or try your exsisting board in another machine.
If you have a spare A3630, fit that for testing (clock jumpers need to be on internal position).
Other devices which directly share the chipmem address bus such as Paula, or the video section could be at fault.
Check for RTC battery corrosion and repair it accordingly.
Use an oscilloscope to check the +5V supply is stable.
That's the first things I'd check anyway....
