starts and runs rough, however after 20 minutes loses power and wants to stall on 1990 Ford Ranger

1990 ford ranger, 2.9l, V6, starts and runs rough, however after 20 minutes rpms go way down, backfires, and will only go 10 to 15 miles per hour. Happens every time. Then if you leave truck off for a few hours will do same; start rough and run until next 20 minutes of running, when power really reduces again. Changed plugs, plug wires, distributor cap and rotor, fuel filter; same problem continues?

Asked by for the 1990 Ford Ranger
Agree with #1, Also in another direction, take a fuel sample and check for water. Water will settle and be picked up again while driving. Check engine oil for gas mix and over full from a leaking fuel pressure regulator. (possible stuff) Good mechanic and an old fashion oscilloscope will be useful on this engine!!
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2 more answers
Have the ignition module tested. If bad replace the module and the stator at the same time.
My chevy had clogged catalytic converter when that happened to me, I was getting oil pass the weakened rings, because my engine got hot when the radiater had an ice blockage.