I just got one of these running for a friend a few months back. The root of all it's problems were related to the AX relay. It was a real fussy one too.

You know I didn't have a schematic to work with. I usually like the shotgun approach. Go through clean and adjust everything properly and it should work. That usually gets you close but you also discover things with that strategy. I found a disconnected wire on the AX and thought that would do it but there was more going on. I seem to remember the "O" relay and the AX being a combined problem with my start sequence.

What helped me most was reading a couple troubleshooting posts related to the start-up sequence for 300 on RGP. I think someone had a similar problem as I did. Posted by the late great Al Garber if I remember correctly.

Guys willing to help with a schematic can usually get you narrowed down to specific areas to focus on. That helps a lot if you want to fast-track it.

The game is "motoring out" as they call it. Fighting to complete the startup sequence but failing some where. Check out the start-up sequence on Clay's site for 1975 and later Gottlieb.
http://www.pinrepair.com/em/index3.htm#start

I don't remember specifically now but this RGP thread helped get me on the right track.

https://groups.google.com/forum/#!searchin/rec.games.pinball/gottlieb$20300$20O$20relay$20al%7Csort:relevance/rec.games.pinball/iCakKmZYAWM/LDGQL4MMVPIJ

These are the kind of things you discover when you go through them completely. All it takes is one of those types of issues to kill a game. Then they sit in some homeowners basement unplayed for 10 years and everything gets gummed up compounding the problem(s).