I'm a novice with Williams boards and would appreciate help is getting this working. The boards are clean and original. I plan to replace connectors (even though I think they are currently not the problem), but I haven't ordered any parts yet.

When I turn on the game it comes up immediately in test 04 and the MPU LED's both flash once and both turn off.

Then if I press the MPU self test button (on the MPU), the LED's turn solid on. If I press the self test button again and again after a few times the LED's eventually flash twice and then off. After that, I am able to use the test button on the coin door...

Using the coin door test button I am able to test all the displays, and they all pass working.
Using the coin door I am also able to test all the feature lights, all the solenoids, and the sounds. Everything all tests fine.

But when I restart the game, it comes up stuck in test 04. What could the problem be?

Just got this board to boot! I had bad 5101. I actually replaced it earlier this morning and had same problem. Replaced it with another and fixed the problem. Guess I had a bad 5101 spare!

Thank you all for the responses!
Jethro