I've flown in lots of different planes with 430's, and I've never seen that type of problem. My experience has always been the opposite; the 430 has been the good radio and the other one (if it isn't a dual 430 or a 530/430 installation) is the flakey radio (if it's flakey at all).

Since Garmin's looked at it and it, presumably, passed their inspection, I might suggest that you find another shop or tech, and have radio bench-checked with you present. If the comm features check out properly outside of the airplane, then I'm afraid they'll have to work backward from the radio tray in the plane to see where the fault may be.