My '93 cluster just started this today. It's kinda hard to explain so I'll try to put it in simple terms. On the interstate rolling around 70-75 I would turn the AC on and the rpms and speedo would go nuts. Speedo would twitch its way to 85+ and peg out, the rpms would read 2500-3500. I have true duels to 4in tips so I would know if I was screaming at 3500rpms, so this is obviously the cluster failing not the true reading. Turn off the AC and it reads normal but still slightly twitches. Blinkers and hazards would seem to make it twitch all over the board with no pattern, and finally as a test I turned the AC off, and rolled both windows down and held the button after they were down and the cluster glitched out again like I mentioned. What kind of electrical issue do I have? Would've got a video but my phone was dead.