Just re read this, the amp is going into protection, basically your running it to hard, headunits and amps are realistically at full power on about 3/4 volume, any more and it starts to distort causing the cut out.
are you sure its the aerial your look at and not the feed to the amplified antenna??my e46 aerial has an amplifier on it so it had to be connected to a switchable ve for it to work properly - this was direct off the bmw headunit.might be the same thing but just a thought