Quoted from orykzinyo:
On your score panel you should have one blank wheel next to the credits wheel and the ball in play wheel. This wheel is what stores the bonus count. Make sure it is operating properly and the edge connector is still snug.
So I totally overlooked the fact that those reels hid mini steppers (kinda) in them! I dismantled that whole left side and cleaned all the old black grease off and added a very light layer of Super Lube. After this I was getting more consistency with the bonus but it wasn't until I loosened the coil bracket on the assembly and pulled it away from the reels some that I finally got the bonus to work correctly! It scores, advances, and resets correctly now!
I discovered another problem afterwards. When the ball would drain I sometimes would lose 2 balls instead of 1. The coil armature had a groove dug in it that apparently was causing it to stick for a second occasionally allowing more than 1 ball to be subtracted. I swapped the armature with the one for the dummy reel and VIOLA! ... that problem is gone!
I have one more possible issue. If I score 3000 points, extra ball light comes on, I drain and I don't lose a ball...this is correct. If however I score 3000 points, extra ball light comes on, and I advance the bonus again (5000 points and special is lit)...when I drain I do NOT get my extra ball. I have tried this at least a dozen times with the same results. I tried moving the plug in the bottom to Extra Ball and to Credit...it doesn't change anything. Any ideas? Maybe this is correct? (I doubt it).