Subwoofer Gets Louder and Quieter on its Own

Hi. I am running a brand new Alpine R-W10D4 subwoofer with an Alpine MRV-500 amp. The amp is at 2 ohms giving the sub a little over 500 Watts (according to the manual this is within the correct RMS range). I have 4 gauge cable running to a XStatic Model 400 batcap and 4 gauge wire running from that to the amp. I then have 10 gauge cable from the amp to the sub itself and all of the cables inside the sub box and connecting the voice coils are 10 gauge (each voice coil is 4 ohms and I have the sub in parallel to make it 2 ohms). My problem is the sub gets loser and quieter on its own randomly and the volume/intensity is drastically significant. I do not know what’s wrong. According to wiring charts all of the wires are the correct gauge. I have no auto EQ or volume feature on my deck either. This problem hardly happens when I’m driving, it happens most frequently when I am listening to my music when I’m not driving (car is at idle). It also seems to happen a few minutes after I stop driving and sit at idle. My alternator puts out 130 amps and the batcap I have is rated for 1,100 to 1,300 Watts. Does anybody have any ideas? Thank you.