1987 S-10 2.8L idle is nice and smooth when first started up but as it warms up the truck tries to die what could cause this?

Double check the timing. If the timing is dead on when it starts and it runs fine that is ok. If the timing is off when it is warm then the vacuum advance is faulty or there is a vacuum leak that only occurs once everything has opened up and the motor is warm. When you set timing on these vehicles there is usually a vacuum line that runs to the Distributor assembly that needs to be plugged when setting timing. This is because it off sets the advance when setting timing. When the engine warms up the Advance opens to advance the timing to maintain time with the motor as it rev's. The higher the RPM the more vacuum that is applied to the timing in order to keep everything clear and keep things from making contact that aren't supposed to.