I have the same set up with a slick mag and Lightspeed Plasma II. On my RV7 the rpm sensor screws in to the side of the magneto. Look for a red hex head with wires coming out of it. If you have the same sensor as me, trace those wires to the firewall and make certain they do not run near any spark plug or high voltage wires. I had an Adel clamp rotate that was clamped on the engine mount and it got too near the ignition wires and produced the same symptoms as you are seeing... with fluctuating rpm.
RV-7, IO-360, BA Hartzell, N847CR
Flying since 2008