I have a 1999 Grand Cherokee, 4.0l I6. Scanned codes and got po123 ( throttle position sensor), p1391 (cam positioning sensor), and a p0352 ignition coil #2. BeforeI replaced the sensors and coil pack it would start and run good with a slight miss, but after 5 to 10 minutes the miss would get worse a few seconds and cut out. Now when it starts it runs rough and blows black exhaust. I've been through 3 coil packs, but when I put the old coil pack back on it runs good with a slight miss but after 5 or 10 minutes it cuts out again.

 

Sounds like your getting bad coils from your source.