Troubleshooting occasional popping from solid state amp?

Identify which channel is involved. If it is both you have a power supply problem. I doubt a cold solder joint as that would have caused trouble a long time ago. I'd be more inclined to look at capacitors. You can replace them yourself by getting the value and rating then you can order the part from Digikey. Do not just heat up the joints and try to pull the cap out. Use a solder wick or sucker and remove all the solder then straighten the wire and gently remove the cap. You do not want to break the copper foil. 

Good luck,
Mike.

Op
I would spray liberally with a good electrical contact cleaner, all the pots the amp has, volume, balance, treble bass etc, whatever as dirt and dust gets built up behind these very easily.
As far as popping when the fridge turns on and off that is interference over the mains from the fridge compressor kicking on and off.
Can you place the amp on a totally different circuit to the fridge?
If not you would need to get some type of power conditioner/isolator for your audio circuit to prevent the pickup from the mains noise.
Good luck

thanks for the suggestions, I noticed now also when switching light on and off it causes popping. It seems the amp has some kind of grounding or shielding issues since I didn't have this problem with other amps? I've read others having popping due to refrigerator. Any idea why the amp could be so sensitive to other electronics?

It may not be grounded or not have a class x suppressor cap across the line. Fitting one may help.

What's weird is that other amps didn't react to the fridge or lights, could be a design flaw?