Stereo Help?

I installed a sub and amp the other day and started having a problem with my stereo fading in and out. It sounds like the sub fades in and out. It worked fine for 2 days now it's acting weird. I'm running a Kenwood dnx7100 headunit. What the heck? I also have the jbl system. I'm still running the door speakers to the stock amp and a separate amp to the sub. 8 gauge wires...

 

Everything works fine most of the time. Actually for a couple of days it worked perfect. But last night it started sounding like all the music fades to a muffled sound with no bass gradually for a couple seconds then goes right back to normal for a couple minutes. I bought everything used from a guy on here, but because it worked fine for a while Im assuming it might not be the equipment.?. Could it be the little amp bypass box atatched to the harness on the hu for the jbl system?

 

What model is the sub/amp. 8 gauge could be a little small