I had a car audio shop move my Rio and amp from my old car to my new WRX wagon. The problem is that now, unless the volume is set very quiet, the audio cuts out every few seconds.

The first thing they tried to do is sell me a new head unit (with free installation!), which annoyed me quite a bit. I was able to prove to them that the head unit wasn't the problem by putting it on AC power and plugging it directly into the amp. It sounded great and there were no problems. When I got home I also put my spare head unit into the car and had the same problems with it cutting out. So I assume the problem lies somewhere between the sled and the amp,

I couldn't stick around to pursue it with them, so I'll be going back tomorrow. I'm no expert, so could somebody give me some ideas as to what the problem could be and what I should be insisting that they check?

Many thanks in advance,
Bill