I bought this truck (99 1500 5.2) a few months ago and had a number of initial issues but now I'm down to just a few items.

When I first start up after sitting overnight or at least a few hours, it seems like it's running on 7 cyl's for a few seconds. After that it seems to be just fine. I changed plugs to Autolite Platinums just a few months ago and the ones I took out all looked good, even #8. And all the bulbs are good but no CEL.
Also, I've hand figured every tank of fuel and I was getting 9 mpg while driving as carefully as I could. I finally said "screw it" and started driving more aggressively (normal) and I'm up to 10.8 now. This is mostly in town driving with about 25% hwy.

Any thoughts on the idle issue? And should I expect any better on the mileage?

 

Have you checked your plenum yet?

These trucks don't much care for the expensive plugs....

 

I bet if you put in some coppers, champions, autolites, NGK, whatever. You will see improvements across the board.

5.2s should get better mileage than that.

Have you done wires, and cap and rotor?

+1 Plenum

 

Cap, rotor, wires and back to regular plugs are coming when I have time. I haven't checked the plenum mainly because the plugs looked so good with no carbon at all. But I guess that will be on the list too.