sub amp help needed

My system's developed a problem. I need help diagnosing and fixing. I have a Kenwood KDC-MP919 deck, JBL BP1200.1 amp, and 2 Infinity Perfect 10.1 subs. The system worked great for 6 months. Then I removed the amp and subs when the Jeep went away to college. After re-installing the amp and subs, everything was OK for a few weeks. But now I get intermittent sound from the subs. The output seems to be related to bumps in the road, so I suspected a loose connection somewhere. The amp shows normal power lights on, but nothing coming from the subs.

I've checked the connections at the amp (OK & tight). I pulled the deck out of the dash and ran a different output cable from the deck to the amp - no improvement. The problem doesn't seem to be overheating the amp, because I get no output when the vehicle and system is cold.

I'd really appreciate some suggestions for diagnosing the problem, even just identifying which component has the problem.

A loose ground could do that, check the ground to your sub-amp and deck. I had that problem too, but it turned out my deck's sub outputs were funky, so its being repaired.

Re-checked all of the connections, including grounds - no difference. The lights on the amp are all lit, even when I'm getting no sub output. Which makes me think I may not be getting any signal from the deck. But I don't know how to check for this. The deck manual says I should be getting 4500 mV/10k-ohm "pre-out level" during disc play. Is this the signal I'd be looking for? If so, I might be able to borrow a suitable voltmeter to check it.

The lights staying on isnt a problem. As long as the system remote wire is connected to the amp while the deck is on, the amp and its lights will stay on. I dont know about checking with a volt meter, Ive never had to do that. Im sure someone else will tell you though.