Possible bad coil?

My 99 Silverado with the 5.3 acts as though it has a bad coil at times,thought it was sparkplugs but isn't.The engine will run fine for sometimes a week at a streatch then all of a sudden it will lose fire on 1 or 2 cylinders,backfire and stumble then come out of it just about as quickly.I am going to get my brother inlaw to scan it for me to see what turns up but I thought I'd try and see if anyone here has had a coil go bad or know of another cause of erratic misfire like this.Thanks.

Could be the coil, but it could also be the fuel injector for that cylinder. Check both. Make sure the coil and injector are properly 'plugged in' to the wiring harness and that niether have loose wires.

Another thing you can look at is the quality of the connection between the coil pack and the engine's PCM wiring harness - if both your affected cylinders are on the same bank and adjacent, the problem could be here. The PCM harness plugs into the coil pack at about the midway point.