intermittent miss

I have a late 84 sportster 1000 xl. It idles fine, but I have an intermittent miss at all speeds, it will come and go. I have new plugs, new air filter, checks and cleaned the fuel system. When this happens the tach goes crazy.

Goes crazy how? Jumps around due to misfire and RPM changes or goes erratic and bouncing all over?

If this is a stock bike you have a dual fire ignition. You will have to determine if it is an ignition failure or a bad plug wire or bad tower in the coil etc.

Take the coil off and look at it carefully. Inspect for hairline fractures at the tower where the plug wires go in and carbon tracking along the cracks or even by themselves. Examine the posts for corrosion or damage.
If the plug wires are original, replace them as they are well over due and they do break down over time.

Disconnect the tach. I have had failing tachs cause weird stuff.

Can you determine which jug it is or is it both? Easiest way to tell is to see if both are running as hot. Spray a little WD40 on the header at the flange. A hot normal running jug will evaporate the WD fast. Repeat on the other cylinder. If both are the same and the coil and plug wires are good you will need to troubleshoot the module and sensor and wiring.

Could it be fuel related? Have you checked for water in the fuel and made sure the float level is correct and there are no large intake leaks?

Thanks for the help. The tach is erratic goes from from hi to lo lo to hi, jumps all over. I have never changed the plug wires. I will try that first. As far as the fuel I ran it dry, and refilled with midrange, and stp fuel treatment. Took the bowl off the carb, and the filter was clean, the float level looks to be correct. I believe both jugs are working okay. I am hoping that it may be the coil, or the ignition module. I just don't want to start swapping parts.