Have a 1990 Corvette, base OHV motor. Engine is running rough and obviously misfiring. Disconnecting #8 injector plug or #8 ignition wire produces no change in engine rpm. #8 injector is getting pulse. #8 ignition wire is sparking. Plug looks good. This car has had numerous repairs in the recent past.... new plugs, wires, injectors, egr valve, etc. No obvious vac leaks. Compression is more than adequate. Fuel pressure is 43 lbs. Any suggestions????