these are good ideas. thanks much.
I know that the fuel pressure is lower than it should be. (was checked recently). Also, it did this 4 years ago, --I took it to a mechanic and he fixed it. I brought it back to that same mechanic and he cleaned out the fuel system and put on a new fuel filter, but it did not prevent the problem–in fact it made it worse.
The exhaust system is a little beat up and I had thought about that as a candidate–but that has not changed since I have had the truck, so probably not it.
I may take it back and have the fuel system looked into more. I will ask him about the ignition module.
thanks again