1987cherokee

I have an ongoing problem with this 1987 Jeep Cherokee 4.0 6 cyl electronic engine control. The jeep will momentarily start then stall and then will turn over but will not start - it has no spark. The first time this happened I went through everything, replacing some parts (ignition, etc.) finally replacing the electronic engine control under the dash on the right side of the steering column. As soon as I did this, the jeep started right up and ran fine. This was a used unit I got at a salvage yard. A few months later it would not start again. I suspected the used electronic engine control that I put in so I went to Autozone and got a re-manufactured one for $99 and the Jeep started right up and ran fine (this had a 1 year warranty). A few months later I was back at Autozone getting another unit and I replaced it and it started again. I've gotten 3 so far...

Did anyone ever have a problem like this? And could you help me find the solution? Someone said it might be a bad ground but I checked them all, something seems to be shorting them out.