Is the Check Engine Light on? If so codes will be stored in the computer of your car and these codes will indicate where the problem most likely lye, (a scan tool will be needed to retrieve the codes using the diagnostic socket under the dash). Vacuum leaks or idle speed control motor are the most likely problem or something that fails because it absorbs engine heat.
To supplement Patrick - if there is not a check engine light on and there are no codes, a good fuel pressure and volume test should be performed. I have seen several of these trucks that have fuel pump issues. Low fuel pressure usually does not set any codes on this one.