Gain Fading Mystery riddle: [D+]

[petemoore]

  DIST +.
  Power supply or battery.
  gain fades out / gain fades in.
  Gain fade seems input sensative, hard hitting chords 'can/may' make the gain fade up or down.
  Pre-boost makes gain HUGE in the D+ until the pre-boost is removed, then the gain will be very low and weak sounding.
  I changed the opamps and electrolytics, the gain pot is 22k so doesn't really do low gain without the mystery gain control.
  Circuit voltages are normal except I'm not sure about the + input, it's at 3.2v of 8v supply.
  I reduced the divider resistors to about 45k from 1meg each, divider reads about 1/2v.
  I twisted it, pulled on wires, tapped on it, twisted the pots, everythings are ok seeming except the mystery gain control.
  Sounds great exept it gets really weak, I'd rather have a Dist+ that has a 'normal' problem which could be isolated, debugged and fixed.