EVJ troubleshooting question

[JR33D]

Hello - I'm new to amp modding/repair and trying to soak up as much as I can. I've done some Valve Junior mods and just picked up a second head to tinker with - thinking about doing the 5C1 mods.

My old head has a weird problem that I'm not sure how to troubleshoot: rehearsing with it one day and the volume dropped to about a quarter of what it used to be. The tone isn't changed much, just super quiet now. Playing around with it, I noticed that when it's cool and I turn it on with a guitar hooked up, strings vibrating, it swells to near full volume as the tubes come on and then cuts down to being super quiet again.

People have suggested everything from replacing the OT to replacing the filter and bias caps on the board. If any/all of these things were malfunctioning, wouldn't the amp be having serious tone issues, blowing fuses, or producing other obvious signs of those problems? Is it possible there's just a bad solder joint somewhere that could be causing this (not sure why a joint would have suddenly gone bad - amp doesn't travel, has worked perfectly for a couple of years)?

If anyone has experienced an amp acting like this and has any pointers as far as how I can diagnose it, I would love to hear from them.

It's the EVJ with version 3 black pcb. It has the BitMO Trio mod installed.