Here are a few items to look at:
The +5V voltage only being 4.8V seems low as when the game starts playing and drawing load, this voltage will drop further. There is a lot of information on this issue. Check out PinWiki: https://www.pinwiki.com/wiki/index.php?title=Williams_WPC
I suggest reflowing all of the connector header pins on the power driver board just to eliminate any potential cold solder joints. Look for any cold solder joints on the large filter capacitors and bridge rectifiers.
I had an opto issue on my TZ under the playfield and it ended up being a broken connector pin; it looked fine until I looked directly into the connector and saw that half the pin had broken off.
I've had problems with the ribbon cables on these older games getting flakey. I generally replace all of the ribbon cables on this vintage of games to eliminate these issues. Pinball Life sells replacement ribbon cables, just check the manual for part numbers.
Here is some music to motivate your repairs: