Some of the previous responses were great, thought I would add to it...
1. CPU power supply. Always check the power supply voltage. Your 5V should be at or above 5VDC (I.E. 5.1,5.2)at the processor board terminals, and should be very stable.

2. Mash all the socketed chips on the processor boards. This has become a common practice with the older MCE boards on jobs with intermittent issues.

2. Intermittent low voltage to the inputs. Some of the landing system boards have capacitors that could be failing. You can cut all or just the related caps off the landing system board. System will function fine without them. They are typically brown rectangle style.

3. Possible failing PI/O or PCI/O board behind the processor. R&R Board. The inputs need either 5 or 0VDC to know whether there on or off. If any of them are 1.5 to 4VDC, the CPU doesn't know what to do with that info. This can be measured at the yellow chips near the center of the board. There are QR sheets out with every pin listed with the description

4. Mac door parts. The rod with the ball socket, too much play in the ball end can cause issues with the lock. The CPU sees inputs going on and off suddenly causing what seem to be bogus faults.