The problem you're having is called 'voltage drop'. It's caused from your PPI amps drawing too much power from your electrical system. The reason that happens is your alternator tries to make more juice, so it turns harder...which in turn, takes away power from your engine, which makes it run at a lower RPM. I guess I'm trying to say you need a new charging system. Get a higher OP alternator, a better battery, and a 1 farad cap for each of your amps and you should be set.