Amp shuts off at high volume

I am having a problem with my amplifier setup and would appreciate any suggestions.  I had an old Sony Xplod (XM-752EQX) amp powering a single sub behind the seat of my Tacoma.  One day it just stopped, for no apparent reason.  It would play but only when I had the head unit on a very low volume.  As soon as I turned it up the amp shut off and the overcurrent light would come on and it would not start again until I shut the vehicle off and restarted.

That amp was about 5-6 years old and given the Xplod's bad reputation I figured it finally crapped out on me.  I replaced it with an Alpine MRP-M350 to power the single sub in my truck.  I hooked everything up and when I go to check it out it does the same exact thing!  The amp works at low volumes but when I turn it up the amp shuts off.

Now I'm starting to wonder if its not the amp but something else since I had the same exact problem with a brand new amp.  I decide on a new battery because the one I had was old and due for replacement anyway.  I pick up a new battery, hook everything back up and the amp worked perfectly....for about a day.

After a day of working fine on the new battery, the next day I have the same old problem.   Turn up the volume - amp shuts off.  Only now its worse than before, it shuts off at ridiculously low volumes.  I'm running out of ideas.  I tried a new ground but still the same problem.  I have a voltmeter but I'm not exactly sure what to check to diagnose the problem.  Any help would be appreciated.

Gain is set to nominal.  I tried setting it lower and I had the same problem. 

Alternator is stock, 1998 Toyota Tacoma 4cyl 2.7L - my guess is somewhere around 70-80A.  Do you think the alternator could be a problem with such a modest setup?  I though upgrades were more for vehicles with multiple high power amps.

The sub is a 10" MTX in one of those vehicle-specific Thunderforms.  Its about 4 years old and was never really pushed hard by the old XPlod and the Alpine only worked on it one day (with the new battery) and the sub seemed fine.  I tried taking the sub out of the enclosure before but after I took the screws out it was being held in by glue(?) somehow and I didn't try forcing it out.

I do have an old box I built for my old truck with a pair of Rockford Fosgates.  This weekend, I'll try hooking the amp up to that to see if maybe the Thunderform is the problem.

Thanks for the suggestions!  Any more ideas?

Thanks for the help guys.

The sub was the problem.  I took out the Thunderform and put in the old box I had, hooked up the amp and it worked great.  Then I took the 10" MTX sub out of the thunderform and tried hooking it up alone with a new piece of wire to see if the sub or the old wiring was the problem.   I tried that combo out and the amp shut off.

Now I need a new 10" sub for the thunderform.  I wish I could use my old box but it doesn't fit behind the seat of the standard cab Tacoma.  Any recommendations for a decent 10" sub?  Otherwise I'll just replace it with another MTX.

This forum is a great resource!