Intermittent Tach Failure

1986 CRX DX in ITC. Tachometer fails to work intermittently. Checked all wiring & ground. Car has aftermarket ignition box, stock distributor. Tach fails at the same time that the clock and fuel gauge fail. In the middle of the race, the tach et al, may run for a lap or two and then quit. Driving me nuts! Almost looks like there is something loading up or heating up to a point where there's too much resistance and then it quits. The real puzzle is the apparent connection to the clock and fuel gauge? I'm getting desperate.

Check all the connections to you instument panel and also the printed circuit board of the IP as I have seen the odd one that has a bad connection that when it heats up there is too much resistance causing it to temporarily crap out and I have also seen the same thing on the printed circuit boards... just a place to start off the top of my head since there is more than one problem...also check if they share the same fused circuit? maybe a problem somewhere else like even a bad fuse connection?