My Amp periodically cuts out speakers until it warms up

I decided to dust off my old amp and preamp stereo.  What I have are a Tandberg 3006a amp, and a Tandberg 3008a preamp.  Both bought in early 80s.  I would say that they had light use.

First thing I noticed is that the speakers would cut out when I turned it on.  By cut out I mean the sound would just stop, then start again several seconds later.  No pop or anything, just stop, start.  After several minutes the cutting out would stop and all would seem fine.

There was an issue with speakers popping when the amp was first powered on.  I replaced the relay switch and that fixed it.  I had hoped it would fix the initial speaker cut outs, however it did not.  

I have tried looking online for a similar issue but have found nothing.

My worry is that the speaker cutting out is a sign of a problem that could become worse or catastrophic.

I know the systems are rather old but looking at the inside of the amp I could see no leaking of capacitors.  I do not know if I should consider recapping or not.

Bottom line.

What could be the cause of the initial speaker cut out when I first power on my system?  Should I be worried about developing issues, or just let it go since it stops after about 5 minutes?  OR.. is it the preamp?