Marshall JCM 900 4100 acting up

[miketbass]

I have a Marshall 900 4100 head on my bench right now doing work for a friend. The power section biases up and all tube voltages seem correct. The amp visited me 6 months ago for a re-tube and got out the door no issues, and has been used heavily since.
Apparently, the OEM output transformer blew out and was replaced with a MM before I saw it the first time. Worked well and then....
At high volume he was experiencing a stuttering volume, which to me said that the tubes were being driven into blocking/cutoff. On the scope I can get my full power with no issues, so I plug in to a 2x12 and dime it to see whats happening.
The Mercury Mag OT appears to be arcing? I can see a spark dead center beneath the wrap when a loud, low note is struck. The power tubes do not red plate at any point. I am thinking that an oscillation in the power stage is present but am really looking for advice on the OT? Or what to check?

[kg]

if it's arced once, it'll arc again. the carbon will see to that.

unless you're willing to partially dissect the wrap and see if you can get at the problem area, the OT is toast.

was the amp run without a load at any point? could have easily flashed over the enamel..

[miketbass]

I was afraid of that... I think I'll work up the courage to get inside the OT.