Random extra balls are kicked out (for example from the upper left popper, or the first gun ball kicked straight back out) even after the trough fixes.

STNG random multiball after the trough upgrades

Cause: Failing optos in the under-playfield diverter tunnels or gun positions that drop out briefly, plus a drop target under the Borg ship not working properly. Optos can test good in switch test and still blink out in play.

Checks

• Clean the under-playfield optos.
• Leave the game in switch test T.1 for a long time; the faulty opto eventually shows as the last switch (switch 33, right gun 2 opto, in the reported case).
• Check the drop target under the Borg ship.

Fix

• Replace the failing opto pair.
• Repair the drop target.

Systems

• WPC DCS

Games

• Star Trek: The Next Generation

Related tips

• STNG won't start and keeps ejecting balls: fuse F103
• STNG: diverter coils lock on during play
• STNG: under-playfield diverter coils lock on during play
• Random multiball or game never ends on early opto troughs
• Random multiball from cracked blue resistors on trough boards
• Turn ends while a ball is still in play in Final Frontier: flaky trough opto
• Cannon won't turn on the third multiball shot though it tests fine: flaky opto wire

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)