Have you checked the cable/wire from your coil to you distributor? My truck would run rough, and lose power every now and then. Sometimes it wouldn't start and finally replaced it and it ran fine after that.

 

You will probably find like I did that the wires going to the primary and secondary coil are broken down. I think GM undersized these wires. Check it by wiggling the wires as the motor is running in the missing phase. You probably get a code too that says primary and secondary misfire.