Mem,
I race a 12 valve for several years. I never had the need to install the heat shield. I know several who did make one for their cars though. Have you tried to check the ECU for any codes?

pure speculation here....Is the ignition's goofy stuff at 3000 rpm similar to hitting the rev limiter? Maybe the ECU is seeing a faulty signal which makes it think it is overrevving..hence the erratic tach signal...maybe...

Jim