Any chance it's an SL30?  My SL30 wired to a Bob Archer wing tip antenna would almost kill the engine when transmitting. My 430 wired to a belly antenna worked fine

I was told by a Garmin rep that the issue is potential ‘bleed’ to different frequency – which is only a potential problem for ATC – nothing to do with any RF noise in your airplane.

I had a similar problem early on with effects on the EC2 by my COM 2.  It was RF being picked up via the OAT probe; but I also was having poor transmission from the com.  Found a poor connection in the antenna lead was causing the problem.  Usually having a lot of RF interference from the radios is due to bad connection or poor impedence match in the antenna causing lots of ‘leakage’.

Al