Troubleshooting / Broken Wire?

[pokus]

Hey there, I have a problem with one of my builds.
Sometimes it acts the way I want it to, but sometimes it does this weird thing.
When the Shape control of the tone stack is in the left position(boosted mids / low bass, treble) there's no difference at all, but when I turn it to the right(mids getting rolled back / bass, treble increases) the volume decreases a lot. So I thought of maybe a broken wire from the dual shape pot that one time makes a connection or not. Which wire could cause this problem? There is another buffer between the tone stack and the volume pot. The only thing they share is gnd (volume1 going to shape6 and then gnd). I actually tested all the wires and always got a connection but as the missing connection isn't constant I wouldn't trust those results.
Or is there another thing causing such a problem?
Thanks for help!