Phuncgnosis oddity (?)

[GibsonGM]

I boarded up a phucgnosis a couple of months ago; now it's about time to box it up!  In testing it, I've been running it before my Dist + - love the sound!  One thing I've noticed, and am not sure how to fix tho, is that if I crank my humbucker into it, sometimes it starts self-oscillating ('farting').  That sounds terrible!  Stops if I roll volume back a bit.  My single coil sounds fine all the time.   I'm wondering if I should increase the input resistance or something (pickup could be overdriving it??), or just live with having to adust my vol.  The 1M pot doesn't do anything to prevent it.
I never did get the 'down' setting working right, so I just hardwired it to run "up".

One mod I might do is to put a LPF on the output, to limit how tinny it can get, just to add some more control to the output.  Or do one of the feedback caps control this?  Any suggestions?   

Great ckt, otherwise, well worth the build time!!