random missfires

Ive got a 95 rodeo with a 3.2 in it and my problem is that the motor starts to missfire, this only occurs if i take it on the expressway for about twenty minutes at high speeds,it continues to do this even at lower speeds as well when i get off the expressway.once the motor cools completely it runs fine again. Its got 75,000 miles on it and it has been regularly maintained by my self. The CEL does come on but it goes away once the motor has cooled, The weird thing is when the CEL comes on it runs fine.I am also getting horrible gas millage even when it does not missfire
This is a list of work/parts that has been done to try and fix the problem and has not worked. Thanks in advance. This problem has been haunting me for awhile and i dont want to dump any more cash into it till i know for sure.

-plugs and wires
-all coils replaced
-fuel and air filters
-new O2 sensor

I'll toss out a couple of idea's.

1. Bad mass air sensor (if you truck has one)
2. Bad battery ground (ground to the frame)
3. The bad gas mileage and runs bad after it is warm might be a plugged catalatic converter?

This may sound bad but i took the cat off awhile ago and just welded in a strait tube. One other thing i forgot to mention is when this problem began my cruise control also stopped working, I dont know if this has anything to do with my problem of it missfiring because the cruise has allways been goofy. Thanks for the suggestions

what kind of spark plugs do you have in it

Things to check as well: 1 Thermostat and the temp sender behind the passenger side head, 2 The vacumn idle motor and EGR valve.
My guess is that your computer is getting a faulty temp read or you have a serious vacumn leak. The map unit would also be a good place to look.

ive got champion platinum plugs in it now but before i had denso plugs in it and it did the same thing.

How can i get the error code off my comp because this damn thing doesnt have a normal diagnostic connector. I have the haynes manual and it shows a three prong clip,under the kick panel. Can anyone tell me what color the wires are and how to get into this thing with a diagnostic tester. Thanks for any suggestions.