Krankenstein problem

Hi everybody-

I bought a Krank Krankenstein used in a pawn shop in 2009, used it very casually for a few years, and sold it last week. The guy I sold it to just called me and was saying the amp would cut in and out very dramatically after about an hour and a half of playing. He said the volume level would fluctuate from almost no sound up to normal level, and continue this cycle. It may have been that way since I had it, as I never played for more than 1/2 an hour at a time. He tried putting new pre-amp tubes in it, and it didn't make any difference.

Any body have any suggestions as to what could be causing this?

Any help would be appreciated.