You are chasing similar problems to what I have been experiencing on my 1989 Suburban - not identical though. If you can trigger the problem by tapping on the computer even after replacement, that starts to sound a lot like dodgy connectors. I used to get frequent code 42's - I solved it by adding an additional ground wire directly from the coil bracket studs on the intake to the firewall to ensure consistant grounding to the distributor. Solved the code 42 for at least a year, but now I am also into a replacement ECU for other problems.
