Every few games the ball ends in the middle of play, as if it had drained and closed the last trough switch.

Ball ends early for no reason: missing switch diodes on the ramp switches

Cause: Missing or disconnected switch diodes. Two ramp entrance switches had no diodes and a T-rex switch diode had come loose, so when both ramp switches were closed together the matrix read a false trough switch.

Checks

• In switch test, close switches two at a time (especially switches that can be hit together, such as the ramp entrances) and watch for other switches showing closed.
• Inspect every switch in the same row and column as the trough switch for a missing, broken or loose diode.

Fix

• Fit a diode to each switch that is missing one, matching the orientation on the neighbouring switches, and resolder any loose diode.

Systems

• Data East CPU Version 3

Games

• Jurassic Park

Related tips

• Trough roller switch diodes get crushed
• "Ball missing" errors or extra balls served (roller switch trough)
• Ball return kicker keeps firing every few seconds: switch bent closed
• What a bad switch diode does
• Use a real ball in the switch test
• Game registers a drain when three certain switches close together: bad switch diode
• Test a leaf switch diode in place
• Using the test EPROM burn-in to find closed switches

Source: Pinball Repair help group!! (Facebook group)