Stumbling during idle in gear

My 355 S-10 recently started to idle rough in drive with my foot on the brake.  In park it idles fine.  It seems to be running fine above idle with no misfiring and strong acceleration.  It has about 15,000 miles on the engine.  The engine is a 355 with Vortec heads, Edelbrock Air-Gap intake, Edelbrock Performer 600 electric choke carb, and a Comp .432/.444 Xtreme Energy cam.  It has  hypereutectic pistons and around 9.5:1 compression.  Any ideas on what could cause this?  I am assuming it is carb related. I have spray-cleaned the carb (especially the idle mixture screw holes), and replaced all plugs.  The plugs looked consistent and fine.  None of them showed any signs off running to rich or lean.  

I am not a expert and you will get feedback from people smarter then I.The engine may have started a vaccume leak.Did the carb bolts loosen up? Check the pcv valve is it sticking?Did detonation ruin a spark plug,too white and smaLL Black spots on it.Do you have a header gasket leaking?You would figure ok it is just a leak.I have had that cylinder not fire and run like crap till I put new gaskets on it.Is there any chance u have watter in your gas?  

I'd start with checking the ingear idle speed, you running a pretty good cam and if you get the revs too low it can't get enough velocity in the intake to scavenge the cylinder on cam overlap which makes the engine run rough.

If not this then we have to start digging.

Bogie

I put a new distrubitor in my bosses ford van last year .It had symptoms of a bad control module but there was a chance the pickup was bad also.Solved that problem set the timming and tightened down the hold down to what felt like 30 ft.lbs.As tight as I wanted to go with a aluminum manifold.It is a 90 e150.Anyway it started lacking power the otther day.I was told to take the converters off and hollow them out.Catylist looked good but did it anyway.Still no power.I went to check the timming agin and I could move the distrubitor.What! So I reset it agin and It is back yo normal.Check your timming agin.! And how tight the distrubitor is.On the van It takes a few minuits to even see the distrubitor.I dont think there is anything worse to work on then a ford let alone a MFI 302 .  

Also I foregot your combo makes a lot of cylinder preasure from idle to 2000 rpm.This is where you dont have much room for a lean condition.Your ignition system is having a hard time firring the plugs at idle with all the compression.I bet you have more like 10 to one with the bore size and the pistons are flat tops I bet.Can you hear it in the exhaust the stumble you call it?I bat this combo makes tons of low end torque and the throttle response must be neck snapping.This is how I want my next one to be.  

Thanks for the advice everyone.  I don't know about the idle speed thing Bogie.  The thing is, I have driven this combo for over two years before it started exhibiting this type of behavior.  I am pretty sure it is in the idle circuit of the carb.  It runs fine and strong above idle.  I will keep messing with it and keep you posted.  To answer your message Gearman: yeah it pulls like crazy in the low-end.  It damn near pulls the front wheels with a stock converter.  I based it on an engine in CHP a few years ago.  I think it was one of the "Seven Spicy Engine Combos" or something.