you have to run a separate ground, ground the wire to the bracket that holds the radio in.

the factory one grounds through the radios chassis. aftermarket goes through the black wire on the radio. or in this unfortunate case the antenna....... if you hold the radio in your hand and unplug the anenna and the radio shuts off.....
you dont have a proper ground.

it wouldnt surprise me if during higher rpm range. the voltage in the system is going up(possibly alot) . since there is such a poor ground, the resistance is increasing inside the radio.