Michael,
I have an XCOM760 - works beautifully, with one, and one only exception. For some reason I lose comms regularly with Brisbane Tower (120.5) when within 3nm of them (overflying at 1500FT normally). The 120.5 aerials are on the top of the control tower - my aerial is on the top of my aircraft. I have a good solid metal ground plane connection, and I have had the aerial (non-aviation style whip antenna) snipped to the optimum length by an Avionics Tech. My aircraft is all metal.
Ideas? (I have considered shielding, but it is the same in varying turning attitudes and distances)
No problems with other Towers - AF, CG, MC, RK, CS, AY, BK, MB etc. etc.