I just spent last night troubleshooting a Quicksilver with almost the exact same symptoms.

Game had been playing great for the last week - no problems at all except for some lamp issues. I installed freeplay ROMS and again, it played fine. I left the game on for a couple of hours the other night and then the game started just randomly tilting during play. Then it started sometimes continuously adding points. Weird.

In switch test mode the following was happening:

SW15 (Left Middle Standup) registered SW15 & SW7 (Tilt)
SW23 (Centre Drop Target- lower middle) registered SW23 and SW15
SW31 (Right Drop Target Middle) registered SW31 and SW23
SW39 (Top rollover button)registered SW39 and SW31

Almost teh same situation as the OP, but not quite.

All the switches are on the same row. Each switch would register its own switch and the switch in the previous column on the same same row.

I thought it may be due to the freeplay ROM so cleared all the audits - no luck. Swapped the 6821's over - no luck.

Unplugged the cabinet switch matrix connector (J3 on the MPU) and the fault cleared! Yay must be something on the coindoor. I couldn't find any issues with the coin door or tilt switches.

Set all the DIP switches to OFF and checked all diodes - no luck.

I'd checked all the diodes and capacitors on the switch matrix and they all tested ok. I then checked the matrix for shorts between rows and columns and didn't find any issues.

I was stumped, so started measuring resistances to ground on the MPU and found that Row 6 was measuring a lot higher (3x) than the others. Hmm. At this point it was late and I was ready to stop for the night. I then came to Pinside and found this thread. Almost exactly the same issue. I just pulled the MPU out and R40 was measuring 182K instead of 56K. What are the chances the same resistor should fail and go high resistance? I changed the resistor and it fixed the issue.

So thanks to robm for starting this thread and documenting the repair process.

BTW I found that if all the DIP switches on the MPU are OFF, the game boots straight into test mode. That had me confused as I though t I still had switch matrix issues.

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