1998 ford escort tach wire over rev

I have installed a Clifford RS1.1L in a '98 Escort. Everything works as expected. My problem is with the tach function. Sometimes the RS will start and run the programmed amount of time with no problem. Other times it will run 5 or 10 seconds and shutdown (usually when the engine is already warm).  On board diagnostics show tach over-rev (two flashes). I have been standing beside the vehicle when it shuts down and notice no perceivable increase in RPM's. I have also tried to re-learn the tach. Still get intermittent shutdowns. I was wondering if I could induce a false learn of a "faster idle speed" by slightly increasing throttle during the learn procedure. Or would that screw up the RS process? Anyone seen this before?

Where are you connecting in the car for your tach signal?  I did a couple of these at a used car dealer back in '99 and I used DEI's information...blue/green wire at the coil pack.

I am obtaining signal at the coil pack (DB/LG). Seems to learn the signal quickly. I tested the over rev by mashing the gas while the R/S was active and it did not shut down. I'm going to change the jumper on the threshold setting and see if that makes a difference. If that doesn't work, then I'll try an injector wire. If that doesn't work, then voltage sense. Bummer to go to the trouble of running a tach wire for more reliable operation, only to have this issue arise. I've read some other posts with similar problem. Thanks for the reply.

Ended up tapping into pin 48 LG/R on the PCM connector. Tach jumper on the Clifford R/S also had to be set to the low setting. Works fine now. (PCM connector is found on transmission hump directly behind console.)