Focusrite pre-pack isa428 problem

Hi all, Recently I got this machine into my shop. It's a focusrite pre amp, high performance studio quality machine. The thing is an intermittent failure on channel 1. When MIC is selected as input, the signal may suddenly disappear. The problems only exist when MIC is selected. This often happens after a couple of hours of operation, so the problem seems to be heat dependent. Also, when selecting another input and again select MIC, the signal is most often coming back. But of course disappear again after a while. I wonder if anyone of you have any valueable tips what could be the problem. Solderjoints are checked. The machine uses low-power relays for audiosignal switching and I may think a relay is causing the problem. Any help is appreciated, Regards Stefan