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...and this leads to the PLA. Yes, the goddamned 906114-01 which consolidates the #C64 address decoding logic and controls all the chip-select lines on the board.

I'm not at all surprised: these old MOS PLAs are known to die often. Though I wasn't expecting that a single output would change behavior while the rest appears to work fine.

Perhaps a single bit-flip in the programmable gates which control boolean equations?