PDP-8 core memory troubles

My PDP-8/A with two 16K core boards has developed a flaky bit on one of the boards. The failure is always in the LSB (11), but there is no pattern to the addresses that I can discern. The +20 volt supply appears to be ok (as is +/- 15 and +/- 5)... it was 19.86v, but adjusting it to 20.00 didn't help. DHKMAD.DG runs beautifully with only the lower 16k installed. But while testing the upper 16k, eventually (30 sec. to several minutes) a random word will fail with its LSB (Bit 11) in the wrong state. Usually 1 when it should be 0 but *not* always! Often this bombs the diagnostic program itself (I suppose if it's self-relocated to the upper field). So far the failures have all been in field 6 or 7. Any tips for checking currents/timing? Or should I just replace the G650, the H219B core stack, or both as a pair? They aren't easy to find these days and my small chassis only has room for two core boards that occupy two hex slots each - so they both have to be 16K boards, as ADVENT requires 32K even though OS/8 runs in 12K :( -Charles