toubleshooting a mtx amp

I have been having some issues with my amplfier lately. It is a MTX Thunder6500D. Here's a link to the product page. First of all, I had trouble setting the gain. My method of setting the gain was to use a 40Hz test tone @ 0db. My amplifer is rated to put out 775 Wrms @ 2ohms so I was looking for an AC voltage of ~40 with my multimeter connected to the speaker terminals. However, I was not able to get even close to 40V until I set the gain past 90% I'd say. I may be wrong, but I think this is way too high. I've checked my ground and it is good (less than 0.5 ohms). Also, my charging voltage is @ ~14.3V cold and ~13.9V hot at idle. I have another 4 channel amplifier (MTX Thunder 404) and had no issues setting the gain in a similar manner, so something leads me to believe that there is an issue with my sub amp. Something else that I cannot figure out, is that the amp will play loud, and have nice punchy bass at times, but after a while it will suddenly lose its punchiness and the majority of its output. After a while it will get louder again, and then quieter, and the cycle continues. At first I thought it might just be me going crazy, but the difference in how it plays at times seems way too substantial for its performance to be considered acceptable or normal. I'll be the first to admit that I know very little about the internal workings of car audio amplifers. I don't know if maybe a transistor has gone bad or something like that, but I thought I would ask on this forum because I know that a few of the members here are knowledgeable in car audio amplifiers. Thanks for your time & help.

i might be able to shed some light on your second problem; some amplifiers reduce output after getting too heated, then once it cools down it goes back to higher power; or, possibly, you dont have enough juice from your electrical system?

does it make your headlights dim?