My radio does not like low temperature. When cold (e.g. less than 50
deg. F), the SSB modulation is bad. Its a sort of brumming noise
superimposed on the audio. Strange, if I change to AM or FM, the
modulation is good, under same low temperature conditions. So, it
seems that the microphone is OK.

What I do now is to heat the radio using a hair drier until the SSB
modulation is OK. In a cold room, I need to do this every 15 minutes
or so.

Any ideas of what causes this, or how to fix it?

Thanks,

Julian