inconsistent sound

I've been experiencing some inconsistencies with the sound quality of my speakers. I have a Pioneer deh-p7800 hu, a Hifonics Zxi 6408 amp, and CDT Es-620z component speakers; the amp is hooked to the battery with 4 gauge wire and is grounded to one of the bolts for the back seats. Initially the CDT's sounded great, and they still do occasionally, but intermittently the tweeters will become overpowering and the midbass will disappear. The change in sound seems to be random, sometimes it will sound terrible right away or will sound good for 20 minutes and then change completely. I used to have infinity kappa perfect 6.1s and an old cadence amp and i noticed this once in a while but just assumed it was because of the weak amp. I don't like the Hifonics amp, but i don't think that is the problem because it happened with the cadence. Could it be a bad ground or an alternator/battery problem?

Check your voltage at you amplifier,  We need readings when you are playing the speakers, (at the level you usually listen) and when the system is off,  take theses readings when the car is on.  It is probably a problem with your source unit.  Something with the Rca outputs.