Truck Repair: 88 GMC intermittently stumbles & dies, vacuum leak, purge valve

Question
88 gmc 1500 w/5.7 liter will stumble when I first start it up in the morning, but clears itself within a few seconds. About 15-20 minutes after driving it runs really rough, feels as if it has a vacuum leak for a few seconds, up to about 1 minute and sometimes actually dies. So far, the truck starts right back up and runs fine. When it doesn't die, it clears up on its own. Any ideas?  

Answer
Hello Billy,
A vacuum leak wouldn't fix itself as it warms up, unless it is the EGR valve being opened for some reason.
Another thing it could be is the temperature sensor. Read this thread.  http://www.chevytalk.org/threads/showflat.php?Cat=0&Number=1685353&an=0&page=0#P...

The vapor emission purge valve opening and causing the rough running after 15 minutes also is a possibility.
Van