Amp/sub cutting out/interference

Hi, I have a few issues with my stereo. I have a Kenwood head unit powering standard front speakers directly and an amp powering a pair of 6x9's and a sub (through different channels).
1, I have had to turn the gain right down on the amp and turn everything on the head unit up, otherwise when I play music loud the amp cuts out untill I cycle power to it.
2, Sometimes i get a sort of whistling interference that varies with the speed of the engine. This normally happens when I first get in the car in the morning or last thing at night, almost seems as if it's when the battery isn't at its strongest. (All my phono leads are run down the opposite side of the car to power leads etc)
3, My sub doesn't start working untill I turn the volume right up then it suddenly kicks in but my 6x9's which run off the same amp work fine.

Any ideas?

Thanks in advance!

#3 sounds like a duff amp - I had the same problem with my Alpine amp after a bit of water got in and goosed it. Could just be one of the filters inside which is knackered, hence why its only on a certain channel/frequency range.

What RCA cables are you using? Its possible if they're too long or not so well shielded they could be picking up interference from the car loom still, especially if the battery is busy sucking lots of power from the alternator. I sometimes get increased noise on the cav when the voltage is low.

As for the amp cutting out on high gain, could be a thermal or load cutout gone awry...

Well the phonos are cheap ones and the amp came from ebay, so maybe I should just buy some new stuff!
Cheers for the input.