Pork Barrel only passes clean sound - sometimes!

[Willybomb]

Gudday all, got a fairly baffling issue with the Pork Barrel in my Space Station build.  It works fine, but occasionally (namely after driving half an hour to praco) I won't get any effected signal out of the PB.  It will still pass unity signal, it just has no wobble.  Kicking and knocking the unit doesn't do anything, but when I get it home and try it out with the little 10watt amp, it works.

At first I thought it might have been an impedance issue as I use a powered PA monitor as an amp, but there's a klon buffer and a buffered splitter before it gets to any amplification.

It has worked at praco, but this is so intermittent I can't seem to replicate the issue.  I opened up the box and insulated the board from everything else - and then it didn't work!  Cycling the power seemed to bring it back on though - this time.

I did manage to get it to fault shifting the board around so I'm thinking loose connection, but I can't get it to fault consistently.

Any ideas?