Welcome to ASR. Yeah that's not normal and way worse than expected.
I see it has a normal volume pot. If your feeling adventurous you could replace with a alps blue pot or disassemble it and clean it (assuming it's the pot thats at fault)
A simple rule of thumb with bridged amps is take the minimum impedance of your speaker and half it. That's what your amplifier will see. So if your speaker dips to 3.5Ω the amp will see 1.75Ω. Most amps won't like that and may die.