Looks like I've got a similar problem on a G master section compressor. Was recapped a couple of years ago, been working fine until recently.
The compressor is overshooting, causing ducking on the fastest attack settings and the higher ratios. It works fine on slowest attack, fastest release and ratio of 2 (the setting most people have it set to!)
It happens when signal is sent to left or right, so I figure a fault on the 27 card or the ratio switch. I haven't got a set of extender cables for the master section so it's a bit tricky taking measurements of the cards!
Anyone ever come across this before? Probably the ratio switch...