I thought berserk was a standard technical term.
On 40m, the side tone, the power meter on the radio, and forward power at the loop are steady.
On 30m, the side tone sounds nasty, the power meter on the radio jumps up and down, and forward power at the loop is very low.
I should have mentioned that the radio behaves itself on 30m when the loop is out of resonance. The problem occurs when I peak the loop (maximum forward power, minimum reflected power, maximum loop current).