Contour Enthusiasts Group Archives Forums Modifications Audio This SUCKS! :P

Sounds like the amp is going into thermal shutdown to me. Whenever there is any high current draw the internal thermistor will shut it down. It gets hot inside where the transistors are mounted to the heatsinks which is where the sensor is much sooner than the outside of the amp will.
This means it can still shut down and feel cool/warm to the touch. Generall this is due to a short, but it can happen when the impedance of the woofers is not matched to the amp. Too low impedance will draw lots of current rather quickly when low-frequencies are present and essentially the amp sees a short.
Look at the amps and verify the operational nominal impedance: i.e. 2 ohms bridged or 4 ohms stereo
whatever it is.
Then verify the nominal impedance of the speakers, 4 ohm or 8 ohm or whatever.
Then add up all the draw on the amp that is shutting down and see if its within specs. It might be a bad amp or a bad speaker coil. Check each speaker coils resistance with the meter. It should not be too far away from the listed impedance. 4 ohm speakers are any where from 2.6 ohms to 4.5 ohms though usually less than 4. In general the resistance of the speaker is generally less than the rated impedance by around 20%. Anyway as long as it isn't close to 1 ohm or something like that unless they are special woofers.

OOH! I almost forgot. Bad connections to the amps(high resistance) will not allow enough current under high draw to supply power and the amps can cut out. Turning down the volume allows the amps to operate again.
Low Battery of course, and low charging system voltage can also cause it.
Check battery voltage with every thing off in the car using a digital voltmeter. Then check voltage at the amp using the same ground point that the amp has. If it differs significantly (lower than the battery) you have a problem.
While the engine is running but everything else off, check the battery voltage at the terminals again with the meter. You should see a minimum of 13.1 volts, as much as 14.5 depending on the state of the battery's charge.
With everything on, headlights on high, ac on with fan on full blast, stereo as loud as it will go and still play, you should see at least 13.1-13.5 volts. Otherwise look at the alternator as well.

Hope this helps, check it all step by step because there is a lot of information here.