[Radioroslyn]

I'd make sure that the base of the bulb and the socket are good and clean. Check to see that the connections at the base of the socket are not shorting(to each other or to the chassis).       Does the volume go up or down by it's self?? Even a little bit?? Don't know which version you have (single ended or P/P output) If you have the P/P output stage watch the filament on those two tube to make sure both are staying lit. If one is intermittent that can cause your problem.

GL
Terry