[sdiy] thermal antagonists

[mark verbos]

Until this year, I have not had many repairs for problems that came  
and went with heat, but I just worked on my third one this year. I  
find these issues very frustrating as they seem to be fixed and then  
come back after the module has been powered up for an hour. Anyway,  
the Buchla 227 that is on my bench right now was killing output from  
the master when it got hot. Using a heat gun, I traced it down to card  
4 when the quad mix buss with 4 vactrols ( one on each of the quad  
channels) controlling the level. Ultimately, replacing the 30 year old  
vactrols solved the problem, but this raises my question...

What parts can be temperature sensitive? I assumed a problem that came  
and went with heat had to be a trace or connector that was shorting or  
opening when the metal expanded. Is it possible that the vactrols were  
at fault or did I fix a solder related problem whilst throwing away  
$20 in Vactrols?

thanks,

Mark