I bought this machine recently, and this was a pre-existing problem. Machine had been unplugged for at least 5 years. Game will occasionally play perfectly for one, or several games in a row.

Occasional error messages:
--Check FU-1 or Field Fuse
--Too many balls found (only? when a ball is already in lock)

Symptoms:
--Occasionally serves a second ball onto shooter lane, sometimes right after serving a new ball, sometimes while a ball is already in play.
--Sometimes when multi-ball starts, locked ball fails to release. Then when only ball in play drains, triggers "too many balls" error.
--Occasionally a drained ball is not recognized and ball in play does not change, But there is already a ball in shooter lane, so same ball continues.
-- sometimes(only? on first ball of game), flippers do not get power until ball makes a score.

Diagnostics performed:

--FU1(on power board) and Field fuse(Under playfield, near lower left flipper) both test good.
--Opto boards removed, lenses cleaned and closely inspected. LEDs light(Digicam), all solder joints look good, pin connectors seem good.
--All playfield switches test good. Tested Optos by blocking beam manually, and with ball in place.

Other info:
--On-board battery had been replaced by previous owner, looks and works good. Original battery was found in bottom of machine. I'll be moving battery off board asap.
--Voltages on all board test points have been checked and found to be in spec.

Still trying to catalog a comprehensive list of symptoms...

--Often If a ball is locked, and the ball in play drains, I get the "Too many balls" error. When this happens, no ball is served to the shooter lane. If I put a ball in play manually the game plays normally until that ball drains, then the "too many" error returns.
--Most of the time, if the machine has errantly served a new ball into the shooter lane during play then the ball in play drains, it starts the next ball, but does not serve another ball into shooter lane (because there is already one there?)