1) Run a small piece of extenstion wire from the battery + and measure volts on the + of the amp. Look for voltage drops as you turn the amp up and it cuts out. Do the same for the - battery to the - of the amp. Ideally, you should always read near zero on this test. If you get a high reading, you have a bad connection somewhere. This test should be done at high volume.
2) Connect a pair of headphones to the preouts of your HU. Either hack up a pair or get a proper adapter. This will eliminate the possibility that your HU is bad.
3) Check voltage on your remote wire when it's cutting out.
... if those tests were good, I'd suspect the amp is bad.
