You could clean the effects loop with contact cleaner. Sometimes they get corroded over time, especially if not being used, and it can cause that. Or maybe it's your mute switch that needs cleaned. Other than that and swapping tubes one at a time to find a culprit, it's about all you can do as a non-tech end user, and if that doesn't work, then it's tech time.
Ampeg Portaflex Club #1