by no means do I mean to hijack this thread, but since the OP's original problem is fixed I thought I would mention a similar problem I am having....very similar actually. Drove to grab breakfast for the family this morning, I DID have my A/C on and whenever my AC is on my RPM's vary back and forth at idle by about 25 rpms, not super rough or anything and other than that it has never caused an issue untill this morning. WHen I pulled up to a stop sign the truck started running like crap, and died. I started it up again and it fired right back up with no issues but was still running extremely rough. I pulled the codes in the parking lot of where I was grabbing breakfast and I got P1693, 1694 and 0230. The 0230 code is always there because I am running a mechanical fuel boss pump, so I'm not worried about that. The 1693 code just means there is a code stored. The 1694 code puzzled me so I researched it and found out it means the ECM is not communicating with the PCM. I went ahead and drove home, it was super rough driving at first but then stopped and seemed to drive fine. I got home and cleaned the 3 plugs going to the PCM and started it up and sofar so good, but since I just stumbled on this thread I think I might be having the same issue as the OP. I honestly thought that was normal for the RPM's to fluctuate a little with the AC on just do to it pulling a little more load on the engine, and besides, it has been doing this for a year or so now and has never stalled out like it did this morning. I'm getting ready to crawl under the truck and see if I can find those wires under the compressor. One thing too I have noticed is a ticking sound when my AC is on, I have never owned another truck like this so I wasn't sure if this was normal or not, and again, it has never affected the driving untill this morning. Thank you for the OP for possibly solving my problem, and thanks to those who chimed in with their wisdom.

 

Just to same some folks from suggesting the usual things to check, my fuel filters are all new, my VP has less than 40K miles on it, fuel pressure remained at close to 20 psi (it will drop some due to RPMs since it's mechanical) even when the truck was running like crap, I don't have any issues with losing prime, it's always started right up and even started up fine today after it was stalling out. Truck is stock other than K&N airfilter, exhaust, and Fuel boss lift pump.

 

I have 99 3500 and I have the erratic throttle sometimes, but mine idles just fine it's when you touch the throttle, it's like I punched it to the floor. I can fire it up and be fine and drive anywhere but once I shut it off and restart it that's when it acts up sometimes. It's hit and miss. Not throwing any codes at all......