Strange problem. Any ideas?

I have an '89 MJ Renix 4.0L, AW4, 231. During the winter when it's cold, it will occasionally start to stutter. This will happen at any speed, RPM, or throttle position, but usually not until the truck is warmed up. It will always clear itself up after a few minutes. Recently, now that it has been warmer the problem is becoming more frequent. I'll start the truck and let it warm up for a few minutes, then drive it for 5-10 minutes and it will start to stutter or lose all power above idle. It has never stalled from this. I'll pull over and rev it and/or shut it off for a few minutes and then it is fine. After that it runs like a top with excellent power and response unless I turn it off for a few hours. At first I thought that maybe it was a fuel issue. I replaced the fuel pressure regulator, fuel filter, and bypassed the ballast resistor. Didn't seem to make a difference. And I don't think it can be the pump or pick-up because it runs so good after it clears up. When I got the truck 6 months ago, I replaced the plugs, wires, cap, rotor, and air filter. It's a great truck, but having to pull over for 5 minutes everytime I drive it is really getting old. What's going on here?

Do you have a temperature gage? If so, try to tell us what temperature this happens at if it's consistent. If you have an EGR valve on your's, have you checked that to see if it's working right?

I don't have a temp gauge, only idiot lights. I have an after-market temp gauge that I want to hook-up, but I need to get an HO T-stat housing first, unless there is another way to plumb it into the motor. There is an EGR valve, and it is hooked-up, but I don't know for sure that it's working properly. How can I test it?

You test the actual valve by hooking it to a vacuum pump - like a Mity Vac for diagnosis. If it holds vacuum, the diaphragm isn't leaking - so no problem, darn it. If it is leaking, it will cause a poor idle when it opens. Simply plugging off the vac line (temporarily?) can help diagnose it, too. No vac leak, good running.

The EGR is opened at certain temps after warm up to dilute the air/fuel mixture and reduce combustion chamber temps, therefore reducing NOX formation, something the HO did with cam overlap.

But it always idles great. Even when it acts up, the idle is never affected, but when I give it gas it breaks-up and loses power. When I let off it returns to idle and runs perfectly normal. Today on my way to work it acted up again, but on my way home it ran mint.

This kind of intermittent shit is a PITA. I always just wish they would die, but at home, so I could get on with resolution. You could take a shot at it being the CKP (crank position sensor). They seem temperature sensitive, but usually not like yours.