Problem with a Soundcraft Spirit FX16

I bought a soundcraft fx16 from eBay and it has an issue with channel 13.
Sometime after power up, after a good while, that channel develops some sort of oscillation that sounds like sssssSSSSSSHHHHHHH. Tapping the top of the board will do little to fix it, but turning off the global phantom power stops it dead and the channel works perfectly. Turning the phantom power back on does not bring the oscillation back untill much later. Then the Phantom power cycle is nessesary to restore the channel to operation again.

What I do know is that the sound is present on the direct out of that channel indicating the fault is near the initial preamp section.

The mixer is one board and SMD - not so fun to work on.

Any insight before I go inside would be helpful.
This is the cue-mix "mother ship" in an active semi-pro studio

:Ron

Well I finally decided I had had enough of this problem to disassemble the board and afaik the trouble was with the ribbon cables that connect the single board mixer to the i/o card. so I swapped them all out with short scsi cables and it is performing as it should now.