I have a Fender Pro 185. It is a solid state amp. The volume seems to fade in and out on it as I play. Sometimes it takes it up to 20 seconds to come back and when it does come back it is usually louder than it originally was. I have discovered that when it goes out for a while, if I press the power switch almost to the point where it cuts off but not quite, there will be a pop sound, and the volume will immediately be back to normal. Any ideas whats wrong with it? I have a gig in a few weeks and need to fix this soon. Thanks
As commented, the best answer is to take it to an amp tech and have it worked on. The volume fading can come from a few different areas, and will require some troubleshooting to determine the fault.
In one case I had an amp that was acting similar to what you describe, and the problem (weirdly) turned out to be a buildup of corrosion across the input jack of the unit's send/return. Electronic spray cleaner fixed it.
That being said, if you are comfortable with basic electronics and opening up your amp, a first step would be to check for loose/broken solder connections, use electronic cleaner on all of your pots and connectors, and tighten all screws and nuts.
If the problem continues after that, then it is probably a failing component requiring professional repair.