'95 Windstar - misfire ???

My '95 Windstar (3.8L) after about 45- 60 minutes driving, generally at Freeway speed, starts to misfire. I can generally accelerate through the problem to the point where the van is running properly again. That is usually short lived as the problem comes back over and over. The weather conditions don't seem to have any bearing. The head gasket was replaced about 15 months ago. Other then that I've had no engine problems. Anybody had this happen? Got any suggestions?

check sparkplugs and wires. I had the same problem engine light would just start flashing and van would kinda spazz in and out. Turns out my 96 windstar had factory spark plugs causing misfire in cyl #1 .I hooked it to the OBD and found some codes P0171 bank 1 lean p0174 bank 2 lean
.As soon as it gets warm again iam going to tear off the air intake manifold and clean my EGR ports cloged from all that carbon, oil gum from the misfire and high use of van the 3.8 air intake is is easy to take off. If you go to ford dealer they will prob tell you that you need a O2 monitor, a O2 senser's, Egr valve, fuel regulator, and try and charge you 2000 bucks and all you really needed was a 5 dollar can of carb cleaner.
6 sparkplugs air filter lots of rags and the wires are optional unless real old.
good luck