amp gets weak the faster i drive

hello everyone i got a problem thats driving me crazey. i got a problem that when the car is off my system booms like there no tomarro. i turn the car on it still boom but a lil weaker. i drive it gets very weak. then if i stop at a light it gets loud again. now i have tried 3 decks 2 alpine an a kenwood same results. ive had this system in 3 other cars no prob. but in this 93 prelude it has this prob. another thing i notice not that im complaining lol but the lights dimmed on my other cars. but on the prelude they dont even dim to the beat. which make me wonder is the amp recieving full power. i got 2 guage ground wire and power wire. ive ripped everything out an rewired same prob.im stomped now. iv never had this problem before exept in this 93 prelude. if this helps im running a alpine cda-9855 deck. an a 500watt rms sony amp model XM-SD22X.

have you ever tried replacing the battery? if you havent i would recommend using the yellow top optima battery. its made so you can push more power to your system. cause it seems like its not getting enough power. but i would recommend it to everyone as it was recommend it to me.

Measure supply voltage at the amps with the system under load (playing hard).  Do this both with the car off, as well as running.  Also test the voltage output of the alternator while under load (at the battery and at the alternator).  If you get a different reading at the battery than at the alternator, check the fusible links for any corrosion, etc.  Check all your cabling to ensure it's clean/tight, especially the grounds.  Let us know what you find.

-------------
My ex once told me I have a perfect face for radio.

i will measure all that. an i forgot to mention 1 big detail. i do got a optima yellow top lol