My Subs Keep Cutting Out :(

For quite some time now, my subwoofer has been randomly cutting out, but not often; just random days. Then the random days started getting a little more frequent. Then yesterday I returned from out of town and when I jumped in my car to get something to eat, it was cutting out the whole time. Today, I drove to my best friend's and same thing... cutting out the whole time. The subwoofer has also been making a thumping sound. Sometimes when it cuts out, sometimes when it cut back on and also on the random.

Since this cutting out mess first started happening I would have to wait till I get to a red light, put car in park and turn it off; then cut it back on and the bass would return. For the past 2 days, when I do that, it just cuts back off in a couple of minutes.

The new thing it has been doing is, because I would have to cut the car off then back on to get my bass back; if the bass cuts off, I will put the car in park in order to turn it off. However, once I put it in park, it will come back on. Sometimes even if I turn it off then back on, then while putting it in drive it will thump and then cut on.

My sub is in the trunk and amp under the passenger seat. I put my hand under the seat to feel for heat. It just felt like regular slight heat; nothing crazy, nothing burning.

I did not do any of the installation. I don't know anything about that stuff. And the folks who installed my sub and amp are different from the folks who installed my speakers and amp. My sub and amp were installed about 5 months prior.

This is ridiculous!... and annoying.
Any ideas?