I have a tweed Fender Blues Deluxe 40W amp. It's not the reissue, but the MIA kind.
Anyway, it's been a good chap for a long time, but recently it's been doing these weird volume swell kind of things, and they're unpredictable. Sometimes it's like loud for a minute, quieter for a minute, and back and forth. Sometimes it's fine for five minutes, then kinda drops out, then comes back at full force immediately.
I took it to the amp repair guy two towns over (he'd fixed my amp in the past), and he couldn't really find anything wrong with it. He put in two new inputs, re-soldered a bunch of stuff, and generally cleaned it up. He couldn't duplicate the problem either.
So but anyway it worked fine for one show (our guitarist used it while I played bass), and then its first time back to work it started doing the swelling bullshit again.
So I just played it for like an hour trying to figure out what the goddamn problem is. It starts off alright... like it went for twenty minutes or so without a swell, then it kinda dropped down a little as I was doing some looping with my El Cap. Then I tried different distortions like a The Elements and an IC Big Muff. It almost seems like the clean tone is totally fine until you start cramming distortion at it... like you turn on the The Elements and it drops, then you turn on the Big Muff and it's fine, but then it drops, then you turn them off, then the volume swells back up after a few seconds. This doesn't happen all the time, but I can duplicate it every once in a while.
But yeah, you have the distortion on and sometimes it will drop volume gradually and the sound gets really compressed and over-saturated, like more than your distortion pedal is. Same with a RAT, Big Muff, Elements.
The best I can tell, and I have no clue, is that it might be the speaker... like it can't take distortion anymore, or like only sometimes, so it kinda peters out, then recharges or something. It's really weird. Do speakers do that? It's a 12", whatever came with the amp.