Stock JBL Sub Problem

MAINLY when playing music CD's through the stock JBL deck, and after about 30-60 seconds from the beginning of a song (usually a song with a descent amount of continuous bass found in a rock song), the stock JBL sub in my truck has a problem. It's hard to describe, but it's like the sub isn't able to sound off, or is unable to play those frequencies. When it does this, it makes a kind of flat, quite, but yet annoying noise. And It's it's not blown because it works in all the other deck modes (FM, SAT Radio, MP3 Player via Line-In), and works sometimes when playing CD's. It just seems to struggle playing music CD's with the above parameters, and it's rather annoying because it makes listening to CD's not that enjoyable. This has occured since I drove this truck off of the lot, and I don't play my music very loud at all. Any idea's would be greatly appreciated?

 

possibly the quality of the songs??? i noticed this too but it was because of the quality of the songs was very poor so that mite be it?

 

I thought it was my sub but it turned out to be the POS rear door speakers