memtest86+ error explosion, and then nothing

It is possible that the memory seating had problems for a while.
You activity with the memory modules get it working again.
I have seen this happen (in exactly the way you describe).

Additionally, there was also a case when the PC would just not boot and declared memory failures. Removing the replacing the memories made it work fine (and completely pass the memtest).

^{I wonder these days if part of the PC cleanup activity should also include cleaning the card (memory and other PCI) contacts... suggestions on this are welcome.}

This sort of behavior can also be caused by a bad power supply or CPU (or even an overheating one that's not actually bad). It could also be a bad motherboard.

If the problems go away after reseating the RAM, than that's well and good.

I was not so lucky, in my case it turned out to be an unstable power supply.