out of nowhere that turn to painful squeals

I can play it for a while very quietly and then this background rustling will start which kind of sounds like static. If I stop playing, hold the guitar far away and pray, it will often die down. If I don't, it will suddenly click in to a louder pitch and eventually it will squeak like hell.

This is a Gemini I G-12, which I believe was made by Ampeg since I've seen the exact same style with Ampeg Gemini II in the used section of Guitar World. The Ampeg tag probably came off or else this was some earlier prototype.

It's an odd amp. It has 2 sides: Guitar 1 and Guitar 2. Both sides have two inputs for a total of four.

On the left side:

tremelo speed knob
*intensity knob
volume 1 knob
*treble 1 knob
bass 1 knob
input
input

-- center = red light when powered on ---

On the right side:

Volume 2 knob
*Treble 2 knob
Bass 2 knob
Echo CH 1
input
input

far right: FUSE (twist to pop out)

Couple weird things that stand out are the *asterisks. Why are the trebles and intensity marked with a star? Is this how they controlled distortion back then?

Then, there's the Echo CH 1 on the Guitar 2 side of the amp.

Then, there's the 4 inputs.

There is a footswitch with about a 3' cord that controls tremelo and echo.

This used to work pretty well at one time, but would always eventually start squeaking and scare me to the point where I thought it would blow up or electrocute me or something. So, I just stopped trying to use it. Recently, I tried again with the volume very, very low and the static/feedback problem seems worse than ever. I can barely hear it, but that rustling starts up regardless of how loud it is.

It sounds like it could be something springy related to ancient reverb/tremelo, but I'm no expert.

Could it be "dirty pots"? How do you clean dirty pots?