Vintage Synth Repair

Hi,

I'm having some problems with my MicroMoog. When I power it up, the 
volume is very low, hardly audible. The volume knob has no effect on 
the volume at all. I can hear the notes I play, keyboard and 
Oscillators seem to work, the filter seems not to work.

After a while, suddenly the sound will appear at full level. The 
Moog will play perfectly as long as I keep it switched on.

Sometimes it can take up to 30 minutes, until the sound suddenly 
appears and it seems that the time it takes is getting longer....

At the moment I keep it switched on all the time in my studio.

Any idea which components could be faulty?

Thanks

UV