When a low frequency exposes something it's generally because it causes a physical vibration that causes the problem to manifest.
Have you tried pounding on the amp with your fist to see if that also causes the problem to happen?
It could be a bad solder joint or even a tube that is reacting to vibration.
hmm... Never thought of that... I will have to check it out...
The funny thing is that I never play in this setting so it does not happen often. however, I don't want it to become a problem.....
Any chance that a tube could be contributing?