Re. the shouting, he could simply be an old COBOL programmer like me!
Anyway, first thing I'd do is pull the head and spray cleaner/lube into the pots, because they could simply be dirty. While you're in there, check for any loose/corroded connections. What I usually do is remove and reinstall every internal power and signal plug I can find. Sometimes they can get corroded. I've even seen some that weren't installed right in the first place. You could also have a problem with a broken circuit board trace (aka "etch"), or a cold solder joint in the process of letting go, either of which would be exacerbated by vibration. You need a magnifying glass and lots of patience to spot those. Or you can use a nonconductive "poker" such as a wooden chopstick to push on internal parts of the amp while you're playing through it. (Obviously you have to run the amp outside its case for this.)