The game starts correctly only on some power-ups.

MPU boots only some of the time

Cause: Marginal reset parts.

Fix

• Fit new reset parts as a set: both 2N3904s (Q1, Q2), the 2N4403 at Q5, the 8.2 V zener at VR1 and the CR5 diode. All must be good for a reliable boot.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• Single reset parts that hold the LED on
• Quick test of reset transistor Q1
• Converting a -133 MPU to a -35
• MPU VR1 is an 8.2 V zener; the manual's "1N9598" is a misprint
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Bench-testing a -133 MPU: swap the R113 diode for a resistor

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)