I have a 1986 Ranger that I have owned for about 10 years now. Recently I resurrected it after it had sat for most of those 10 years. There were a number of small things wrong when I started this process and I've been able to fix most of them. Here is a list of the things I have replaced so far since June 2010.
spark plugs
plug wires
cap & rotor
fuel filter
fuel pump
fuel pressure regulator

Now most of these things I thought would help with the problem of it misfiring, but the problem persists. It will start well and run for 15-20 min or 10 miles out of town and then start to lose power and miss. The last thing that I did was to take care of the catalytic converter to allow better flow of the exhaust, but this didn't alter it one bit. This truck has 114,000 miles on it and the motor as far as I know has never been touched. Also, when I changed the plugs they looked really good and not burnt at all. Does anyone have any thoughts and have experienced something similar with this engine? Any help appreciated.

 

Sounds like the ignition coil is breaking down after it gets warm.