I've got another idea. I recall troubleshooting a Mutron III a while back. Turned out one of the opamps was oscillating at high freq. If this board was on my bench, I'd put a 'scope on the opamp outputs. Since you don't have one, let's see if some band-aids will fix things. Tack solder a 470pF cap across R7 and another one across R5. See if that makes any difference.
Tried this. While it got rid of the fizz and wobbles, it killed the sweep. Basically just sounded like a wah stuck close to the heel down.
There really should be a small resistor in series with C4, something around 100R would work. I've seen this time and again. Whenever D1 or D2 conduct, C4 and C5 load the opamp's output and that is a recipe for oscillation. It's sloppy circuit design and completely avoidable with one resistor. Before adding a resistor in series with C4, just remove C4 and see if you can tune the filter back & forth with the LEVEL control. If the sound is good with C4 removed, then put it back in with 100R (or something close to that) in series. You will have to get creative with how you accomplish that, but desperate times call for desperate measures.
I'll try this next. Does it matter if I replace C4 with a non-polarized cap?