Customer (name blocked for privacy);

On the 4.3 vortec engine, besides the common fuel spider assembly problems, theer are other very common failures that will cause symptoms like you are experiencing:

(1) The fuel line assembly connecting between the fuel spider and the outside of the engine commonly develops pinhole leaks, resulting in fuel leakage inside the plenum. If the shop that replaced the spider did not replace the fuel line assembly inside the plenum also, this is a definite possibility.

(2) The most common idle problem on these engines is carbon deposits keeping the EGR valve from closing, creating an internal vacuum leak that plays havoc with idle. This is very easy to diagnose if you ahve a GM TEch-2 or equivalent scan tool that allows you to read EGR position and command actuation of the valve. If you do not have access to such a tool, remove the EGR valve and look at the pintle area; you will probably find little chunks of what looks like charcoal holding the valve open.

I hope this is helpful; if so, an accept would be most appreciated! Thanks!