My Christmas present to the children was a Commodore 64.

I picked it up a couple of months ago, and ran it for several hours to
test it; it was all fine.

On Christmas Eve I tested it again; after a few minutes it would start
throwing junk all over the display and then lock up, and I realised that
the difference was the house at room temperature where it crashed, and
the much cooler garage where it wouldn't.

We've repaired it now (mostly), by resoldering many of the pins on the
PCB where the solder seemed to have developed holes. I've never noticed
this phenomenon before, even in much older equipment. I assume that it
wasn't like that after manufacture.

What causes it?

Presumably, a 30-year-old computer would have had proper lead solder,
not this tin-whiskery lead-free stuff.

Actually it can't have been the soldering alone that fixed it because it
continued to crash; I also used a hot air gun to try to identify the
component that was failing after warming - I only once managed to get
that to trigger the fault, but since then, it hasn't crashed again.

So the crashing is fixed, but the next thing to tackle is the sound
output, which is barely audible.

Anyway, the children have been happily playing The Hobbit and Attack of
the Mutant Camels from tapes that have been sitting in boxes for nearly
three decades, which isn't bad going.

Daniele