Please Help! Problem with TA-30 combo

Hi all. Im in need of some advice.

My TA-30 is doing something very weird. About 30 mins into the first set last night it started doing this thing where it's like a power surge, distorting really badly, with a massive volume spike, but just briefly & intermittently. The sound is like a blown speaker, but it can't be because when the surge isn't happening, it sounds perfectly fine. I thought it would be a bad power tube, but i checked them today and they appear to be ok.

It happens on all channels and on all the different wattage settings.

It's hard to explain and not something i've ever dealt with in an amp before. I haven't been able to reproduce the issue at home but it has happened at the last 2 gigs now.

I've got a spare set of JJ's, so i think i will put them in and see how it goes tonight

Could really use some help, this one has left me scratching my head

Please ask questions if you would like me to elaborate on the symptoms.

Changed the EL84's, but the problem is still there. I have worked out that it is only happening when the guitar volume goes above about 3/4's or so. When i turn it down, no problem (but no tone either!).

Ok, I've worked out the problem is related to the reverb. When I turn it down to zero, or bypass it, the problem disappears. Turn the reverb on and the problem reappears.

So I'm not sure what to do now. I've never played with reverb tanks before so I'm not sure where to start.

Any direction you guys could offer would be much appreciated.

I live in a regional area & unfortunately don't have a Mesa repairer close by.

I'd suggest replacing the tube that drives the reverb.