I had a somewhat similar problem on my 940S on transmit....it began by
showing unstable output on 15m when operating FSK (i.e. power output would
fluctuate) then started to spread to other bands.

I re-seated every connector on all the boards I could physically get to and
used a little bit of DeOxit when putting them back. Power output is rock
solid now all bands, all modes.

Who knows where the problem was....I'm just glad it's gone! Hopefully my 18
year old pal will keep going for years (although I do threaten it every once
in a while by showing it a picture of the new Icom 7800 just to let it know
who is boss!).

Dino KL0S/4