It seems that every electrical problem with my Z has been resolved by either putting in a better grounding strap or tightening the original one. I would suggest you locate every last one of your grounding locations and make sure it ready is a good ground (connection is tight, nut/bolt not rusty, wires in good shape) then tighten. Add a 8 gauge strap from the engine to the frame and you should be fine. Remember that there are several grounding locations on the frame (fuel pump- electric if you have one, lights, wiring harness along passenger side, lights, horns, etc) and may in the interior (the center console has at least one major one). I don't know what you do for a living but whatever it is you can't afford to pay some one to do this (I worked for over 40 hrs finding 2 bad connections in the dash. At $50/hr that'd be $2,000. just to fix a grounding issue). Get a test light and voltmeter and practice getting flexible since you'll be twisting yourself into small places under the dash, etc. A small foam pad is good for your knees. Good Luck......be there and done that.