A little patience and persistance will go a long way with this problem. Remember, your Z is almost 40. It's due it's aches and pains. Remember, it's not an American car with just one common ground. Your grounds are just as important as the hot wires. When my 77 stated having problems, I just made it simple. I got rid of the FI and all the wiring. I rewired the whole car with a litle heavier gage wire. I used relays and every connection was new. Ford makes an anti corossive grease that's awsome. Disney land uses it in all their electrical connections. Then, I spray Aqua net hair spray over all my connections. The USAF uses the same spray on their jet aircraft and call it electrical sealer. I know, I was a jet aircraft Mechanic during the Viet Nam war. Scrape off the OD green paint and you see the pink aqua net underneath. My point is: make sure ALL your grounds are clean. It sounds like you are getting some backfeed somewhere. Also, you can clean and regrease your T/S switch. If you swap it for another used one, who is to say it doesn't have the same problem? They are fairly easy to clean. Make sure your battery to body and engine to ground is clean. Hope this helps.