My heartfelt advice: DON'T GIVE UP.
I have only been in the hobby for six years, and the frustration that you are describing has already happened to me many times. I also expect that it will happen to me many more times in the future as long as I keep the damn things in my life - and I'm pretty sure I will keep them.
Most recently, my 1975 Williams Pat Hand was down for at least 6 months. I put up a post here at one point and got some help, but EMs are sometimes mysterious and addressing one issue led to others. I gave up and had Clay Harrell to my house. Clay is an AMAZING person to watch work on a machine. I have paid for him to come out three times in my six years in pinball. Early on (probably visit #2), I stopped trying to learn how to "fix" something from him and decided to try to learn HOW he assesses and works through an issue. I found this to be a much better way of trying to get something out of his visits. He is ALWAYS helpful, funny and committed to getting things fixed. In my own way, I have learned a LOT from him by watching him work his way around a pinball machine, especially an EM.
When Clay left last time, Pat Hand was working fine. Within 48 hours the same problems were back. Instead of losing it and getting all worked up, I just kept working my way through the schematic the way Clay did. I elicited a little help from an engineer and an electronics mechanic during a pinball party at my house last month. The night after the pinball party, I opened up Pat Hand, took a deep breath, and started chasing down circuits and checking switches. Next thing I knew - BOOM - back on came my playfield and I haven't had an issue since.
So in the end, I know what I did and I know how I did it. Sure, it took MONTHS and a payment to Clay to get things back on line. For me, it means 2 things:
1. I have learned a little more about pinball repair and have a couple more tools in my mental arsenal the next time an issue comes up.
2. I have learned that the next fix - whenever it may be necessary - might be just as confusing, anger provoking and head scratching as the last. Or, it could be fixed in 10 minutes.
Either way, I will keep learning and improving my repair skills, as long as I keep trying.
Keep at it! Don't give up! Don't worry about how long it takes. Just keep trying. At some point, things will work correctly and you will be standing there playing a game on the machine again and - from what I have experienced in the last six years - it will feel pretty damn good!
Best of luck!
Tim in Motown