Ibanez SM-9 punch stage problem

[Gix]

hi, i have built sm-9 and the sound is good
but when i turn over 50% the punch control
i have an auto oscillation with a big low note at output.
this stage work in this mode with 9v battery,power supplay (ua7809),
out and in the metal rack and also with input shorted to ground.
can i solve this problem?