Subwoofers Messed up?

So i have 2 10'' alpine type R woofers in my pontiac grand prix with an alpine mrp-m500 amplifier. i dont have a capacitor or anything fancy really. anyway it plays well until i press on the gas, when i press on the gas to accelerate the b gets much quieter than normal and when i let off the gas it plays normal. i have no clue why this is happening and neither does anyone i ask. would a capacitor help? or maybe an amplifier upgrade? i would think pressing the gas would generate more power but apparently not. please help me out