while sitting at a light or stop sign the RPM usually sits around 600 RPM. Lately it starts sudenly jumping to 900 and then down to 300 RPM. the truck has stalled and i had to restart the truck.

May be the Air Idle Control sensor, they will get gummed up and the port that it is in would need to be cleaned out good, vacuum leaks are another thing to check, you can spray some carb cleaner around the intake gaskets, throttle body & hoses, RPM will pick-up if there is a vacuum leak, then timing should be checked, and the fuel pump pressure, after replacing the fuel filter.

Rowefast is correct about the iac and vacumn issues, If you are using the defrost when its happening you are making the engine work harder due to the a/c compressor turning on and off.

I have a 99 and experienced the same thing. First, I removed the throttle body and the IAC and cleaned them well with throttle body cleaner. Believe me, they get gunked up with carbon. After cleaning, make sure the IAC works and moves freely in and out with the key. Nine times out of ten that fixes the problem.