what work was done to it . does the jerking stop after the engine is warm I have a 1992 K1500 that I have done much work on through the years including pulling the engine and reinstalling it after it was rebuilt (Cracked Head). Anyway, I would suggest that you look at the Throttle Position Sensor (see explanation below). When the TPS is functioning it will increase the idle speed when the engine is cold (especially when the engine starts). You will notice it more on a cold morning that the engine idles a lot fast right after it is started but slows down a few minutes later to the normal speed of around 1000 RPMs. If this sensor is bad, the engine can idle too slow during cold times and die when you take you foot off of the gas. You may also want to have the timing checked. (http://autorepair.about.com/cs/fuelinjection/a/aa030301a_2.htm) Throttle Position Sensor (TPS) The throttle position sensor responds to the accelerator pedal movement. This sensor is a kind of potentiometer that transforms the throttle position into output voltage, and emits the voltage signal to the ECM. In addition, the sensor detects the opening and closing speed of the throttle valve and feeds the voltage signal to the ECM. The ECM receiving the signal from the throttle position sensor determines idle position of the throttle valve. This sensor controls engine operation such as fuel cut. On the other hand, the "Wide open and closed throttle position switch", which is built into the throttle position sensor unit, is not used for engine control.
Check for vacuum leaks that occur only while cold especially at the throttle body gasket. Check for low tranny fluid level.