Also the gasket usually just lets coolant into the intake runners, or leakes out. Just a thought! Check your codes, and then check the amount of fuel pressure at the rails. Then get a spark tester, and check eack plug, that should help to tell if it is your coil. Weak spark, or weak coil would cause the problems that you mentioned. If the fuel and the spark are good, I would say it would be a computor thing. And that is beyond me.

 

I had the same symptoms on an 88 5.7 a few years back and what I found it to be was one of the baffles in my mufler had broke loose and as it got hotter the baffle would restrict the flow. Unhook the exhaust in front of the mufflers and run it down the road to see if it runs better, if so then you know what the problem is.