Beefcake35

[Had a new amp and stereo put into my boat. JL audio amp. When the boat is not running everything is fine. Sounds awesome. When I start the boat I get constant buzzing. When the rpms hit 1200 the amp shuts off but the radio stays on. When rpm goes back down amp comes back on. I have checked all connections to battery etc. thought it was maybe the alternator so I replaced that. Still doing it. Any ideas? I am flustered. I checked voltage coming out of alternator at higher rom and it all checks out good.

thanks in advance!!