amp cuts out while driving

stats:

2002 Jeep Liberty

Sony head unit(don't know model number.  2 sets of preamp outputs, 1 aux in on the front, Ipod cable attached to the rear).

MTX pro thunder 504 amp(old amp)

amplified tube(cheap brand)

pioneer 6.5" front, kenwood 6.5" rear, factory speakers in the dash.

Amp front/rear signal coming from head unit rca outputs.

Amp located under driver's seat.

Sub signal coming from radio front left speaker output( doesn't have rca input)

Problem:

Owner of vehicle complained about music intermittently cutting off in the car while driving.  Sub continues to play when this happens.  I looked at the jeep today and couldn't duplicate the problem while sitting still.  However, did notice the front driver's side speaker cutting in/out at a certain volume.  Swapped the speaker with a new one, and the problem stayed.  Swapped the L/R speaker outputs at the amp and the problem stayed. Disconnected the speaker wires from the amp and checked continuity on each wire and to ground.  All checked out good.  Checked wires from amp to radio( tapped into wiring harness) and didn't see anything unusual.  Re routed wires anyway and put everything back together.  Didn't have an issue.  Didn't have an issue while I had the seat and amp unbolted and carpet up to reroute the wires.  Pulled radio out(mounting bracket and all) and didn't see anything pinched or anything behind the radio. Checked connections at battery, and the fuses.  All good.  Checked voltage at amp on B+ and turn on and both are above 12v with the jeep off or running.   Owner is ok with the situation.  Owner leaves and calls within 5 minutes and says it's doing it again(sound cuts out in all the speakers).  Called owner around 15-20 minutes later and he states it hadn't done it since then.  Previously, I had the owner check the amp to see if the light is still on or not when it happens and he said the last time it happened, the light was still on(power should be good).   The owner did say that the amp was old(which it is), and was out in his toolshed.  My next process of troubleshooting is to wire the speakers directly to the radio and see if he has any problems.  Oh, during this whole process, the sub doesn't turn off.  Anyone have any ideas?   I don't think I've ever seen this one before.  I've tried smacking/banging on the amp to see if that was the problem also.

I don't think that's the problem.  I was having an intermittent cutting off/on issue out of the driver's front speaker while I had the speaker out of the door.  I checked where I soldered to the factory wiring and it was fine.  That's what I was originally thinking also, but once you turn the volume up alittle further, the problem goes away.  When this happens, the volume is actually not up too loud.  Around alittle less than halfway on the radio.  I've done installs for years, and I haven't seen this one before.

I'll check that.  I have the amp in a bigger 2 channel version and don't have any problems.  I did rev the jeep a couple of times to 2500 rpm while sitting in the garage and it didn't make any difference.  I'll still check it though.