turn off and on

Well I’m lost, my hu has been shutting off once the bass hits but turn right back on, however if the truck isn’t moving and Is in park it won’t shut off even if I turn the music up (same song) not sure if its a lack of power from the alternator or a bad ground what do y’all think ?

So the system is 2 10 inch alpine type s subs with a 1800 watt hifonics mono amp (wired at 1 ohm 1000 rms), the problem is that the subs will randomly go from working with full power to barely thumping (just moving enough so i know they are recieving some kind of power and also worth mentioning...