I have a problem with my idle. When I pull up to a stop light my idle goes way down. I tap the foot feed and the idle comes back up to where it should be. Could it be the dash pot?

 

I don't see how, the dashpot only slows the throttle closing, it doesn't affect idle speed after the throttle is closed. I think the idle is controlled by the IAC valve.

 

It is possible that the dashpot or throttle linkage is sticking just enough to prevent the TPS from sensing the idle position.

If that happens, the ECU pulls your rpm's down to 500rpm.

The TPS has a knife edge off-on for the throttle open-closed circuit in the TPS. The TPS could also be slightly out of adjustment.

You can check this by disconnecting the TPS connector. If your truck now idles and runs correctly (this will throw a code which you can reset after plugging the connector back in), then one of the above is likely to be the problem.