I don't think It's about ur subs drawing to much power at all I have 2 10s with a 900 watt amp and my system just barley makes the power in the car fluctuate so I'd say look through your wiring to see if one of the wires is starting to corrode also clean off the wire going into the alternator I constantly do that with mine as well as the ones going into the battery but again if it's power related u can always wait till night to do the test and turn on ur head lights and see if there is any fluctuate in power
