GG is right, this sounds to me (and I've worked in electronics since 1984 and currently work for JVCKenwood) like you have either a bad amp (It happens) or you have a bad load on the amp - is there ANY chance that you have one or more speakers not working or VERY low volume (this would indicate that maybe one side of the line feeding those speakers is coming electrically close to ground or the voice coil of one speaker has a shorted turn).

I'm sorry, but it may be hard to get to the bottom of this one! I actually have a spare OEM amp here I could send you, it was pulled from a car and given to me but I am going to go all JVC with my system (since I can get free stuff from our marketing department).

You could swap the amp, if THIS amp does the same thing, then you have a speaker/wiring issue. If this amp works fine, then just throw yours away!!!

That's as good as I can do.