First, have you installed the grounding screw in the remote cable of the Icom? If you haven't, that's probably why you're having the problem. In extreme cases, you'll have to install beads on both ends of the remote cable. It will take at least four turns through a mix 31 bead, so use a 3/4 inch ID.
Secondly, the ground side of the coupler MUST be made as robust as you can. It must be inches, not feet!
Next, the feed side of the coupler must connect directly to the antenna. You cannot use coax for this connection. It too much be short, inches, not feet.
Next, remember that every piece of hardware past the coax connection inside the coupler is part of the antenna. If this is inside the plane, which I'm sure it is, you may not be able to cure your problem.
Lastly, the coax most likely is acting as part of the antenna, and you'll need to choke it off too.
I used an SG235 in a vehicle for several years, and finally gave up as I could not tame the RFI. And trust me, I tried EVERY trick to do so. The short answer is, the only way you'll ever pull this off, is to have the coupler on the outside of the aircraft, and even then you're going to have problems if you don't make all of the connections robust.
If you want more info, read my web page article on Antenna Couplers.