OK guys, here'e the root cause of the problem and how to fix it:
The little black CDI unit creates a very short duration, very high voltage, and relatively high current pulse that feeds the spark plug. This short, high-intensity current pulse travels along the spark plug wire from the CDI to the spark plug. Whenever you have short duration pulses of high current in a wire, the wire radiates an electromagnetic field that can interfere with nearby sensitive electronic equipment. You might be familiar with this when a hot-rodded car pulls up next ro your car and you hear his "ignition noise" in your AM radio. This is produced by the same mechanism, but in our case, the noise interferes with the computer's ability to receive the small signal that the wheel sensor sends to the computer over the interconnecting wiring.
The solution known for years to silence ignition noise is to put some resistance in the high-voltage wiring, or inside the spark plug. This limits the peak current in the spark wiring to a level which attenuates the amount of noise radiated, but it also limits how 'hot' the spark is. It's a tradeoff.
My Target speedo computer would reset every time I started the engine, and often when I was riding too. To prove my theory, I went out and purchased an Autolite 425 plug, which has a built in 4000 ohm resistor. Voila! The interference problem disappeared! Unfortunately, due to the weaker spark, the engine didn't run very well until it got good and hot, and even then its top end under load wasn't as good. I figured that 4000 ohms was probably a little much for the amount of energy our little CDI's put out (as I said, it's a tradeoff), but at least I proved what the problem was.
My next approach was to go back to the original plug and replace the stock spark plug wire with some automobile spark plug wire (this stuff usually has a resistance core). I measured the resistance of a 2 foot ignition wire I had laying around and found it to be about 2000 ohms. I figured that if I cut it down to about 8", the resistance would drop to about 600-700 ohms or so, which was right about where my gut said I'd like it to be. This is a huge increase over the original wire which was only about 50 ohms, yet much less than the troublesome 4000 ohms built into the resistor plug. As expected, the bike now runs as good as it ever has, and my computer is 100% reliable.
If you do this mod, here are a few things to keep in mind:
1) You want a 7mm diameter automobile plug wire that has a right-angle cap on the plug end.
2) You'll have to install one of those little screw-on metal connection tips that come with new spark plugs, since your stock plug only has the threads and no tip for the new plug wire to mate properly with.
3) Remove the original wire by unscrewing it from the CDI body. It comes out pretty easily.
4) Cut the new wire about 6-8" from the plug cap end (or whatever length you need to reach from your plug to your CDI).
5) Push the cut end of the wire into the CDI body while turning in a clockwise direction. The CDI has a pointy screw sticking up inside that will thread into the core of the wire. Once the wire has gone in about 1/4", you're done - Don't over-tighten!
This should fix your problem. Let us know how you make out
Joe
