Car Stereos: Subwoofer Static, bump in the road, listening to the radio

Question
I recently had an aftermarket head unit along with a 12" subwoofer installed in my car.  It was done professionally and lately I have noticed two things: after a few minutes of listening to the radio or CD the subwoofer, at an instant, sounds as if I have turned down the base. It goes from booming to barely on. It still works but the base is really low even thought I have not adjusted any settings.  The other thing I've noticed is when I hit a bump in the road there seems to be a small amount of static coming from the subwoofer for a split second. I checked the ground wire from the AMP and it seems to be securely screwed down.  

Any suggestions are greatly appreciated.

Answer
Hi Marc,

The bass going away sounds like the amp is overheating and going into protect mode.  Once it cools a little, it'll flick back on (which can happen a lot if it spends some time right at the brink temperature where it shuts off.)

The static I would suggest is likely to be a speaker wire lead touching something it shouldn't, or rattling around in or on the speaker connections.

Justin