This problem is easily solved. Get yourself a relay and instead of using the wire that feeds to the black wire for sensing purposes for just that, use that wire to trigger a relay that will turn on a much shorter (fused) wire direct from the battery. That way you eliminate all wire length and corrosion based voltage drops in the current stock wiring. That wire circuit goes from the battery, through the 30 amp fuse at the starter relay, up to the ignition switch, back to the fuse box through another fuse and only then to the black wire on the VRR. That's a lot of length, to say nothing of all the connections that can oxidize and corrode. Use that circuit instead to simply trigger a relay and get a more direct and fresh feed straight from the battery through the relay.
Wire as follows:
From positive terminal to fuse (5 amp is fine - it's only sensing voltage). From fuse to relay. From relay to black wire on VRR.
Use the wire from your stock wiring that used to go to the black wire on the VRR to trigger the relay.
If that doesn't take care of the problem, something else is going on.
