Please help: Amp turns on for a sec then turns back off
 

ok, so I have an Alpine MRP-M500 amp and when its off its at about 12.5~ volts , when i turn the amp on its drops to 5~7 volts. But, as soon as i turn my radio off it stays at 5~7 volts til i disconnect it from the batt. So, could i have a grounding problem internally? Cause the ground to the amp is fine and so is the power cable and it is a 10 gauge cable. Thanks!