Obtain or build a class A amp (from a kit) and leave it on all the time 
(without signal).

Alternatively, obtain the schematics for your amplifier, determine the 
feedback circuit (one of them) and see where a thermistor could be 
applied to fix your problem.

Chaning gain to that extent (10db) over a short time indicates a failing 
amplifier, maybe dried electrolytic capacitors. If the amp is older than 
about 10 years from manufacture then this must always be suspected. The 
fault can be duplicated using a heat gun. You can measure output in db 
using a normal d'Arsonval meter on db scale with a dummy load.

ime an amp in good condition will not change gain by more than about 2db 
from storage-cold to stage-boiling.

Peter
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