Ummm power caps are a total waste of money, really just a band aid solution.
Are you running the power for the amp directly to the battery?
If so then it sounds like you have either your battery on the way out or the alternator. How old is the battery? They only last about 4-5 years so I'd start here as it's a relatively easy & cheap fix.
If you're running the amp power off the heavy duty terminals under the dash then this is more likely to be the cause as other components run off these connections and the wiring won't be rated for a heavy current draw for an amp, so you'll get a voltage drop on both the amp and any ancillary components running off that supply.