Troubleshooting occasional popping from solid state amp?

Hi,
 First disconnect everything and try to identify if this popping is coming only from the amp. Cold joints are not always easy to trace and you have to dismantle the unit to check thoroughly but normally they keep the unit working or not. 
A 20 year old product needs some attention if you want to keep it.
Pops like that are sudden short bursts in the circuit. Better checked.

Open up amp on bench. Connect to small test speaker. Power up. Use insulator like thin dry wooden rod to push on board and connections until faulty joint is found. Use freezer spray too. Look for leaky electrolytic capacitors. Consider replacing them.

What I noticed is that the volume pot makes a pop when it's touched, could this mean the pot is the problem? Could be a grounding problem of the pot? Any advice on how to check it?

Yes. Could be a bad connection on the pot. Replace the pot - or, try contact cleaner spray if you're happy it's not a bad joint.

I noticed also the amp pops when the fridge turns on and off, any solution to this?