Hello folks, hoping for a little help here....

I'm having a rather strange issue with my 01 1500 5.9L...there are no codes and here is a brief list of what I have replaced/done since I bought the truck in Sept. 2012:
Full tune up (plugs, wires, cap, rotor)
Plenum fix (hughes airgap - relocated IAT sensor to K&N air tube)
180 t-stat
New coolant temp sensor
New TPS and IAC (delphi for both, computer reset after replacing)
New battery and alternator
New fuel pump (delphi)

The oxygen sensors, MAP sensor, crankshaft position sensor, ignition coil, and catalytic converter are still factory original.

Now for the problem - approximately 1-2 minutes after I start the truck (right as the idle is coming down to around 800-900 rpm), the rpms will begin to fluctuate rapidly and the truck begins to run realllllly bad and act like it wants to die. If I give it gas, it does not want to respond to the accelerator and continues to bog down. This will go on for maybe 30 seconds to a minute until the engine becomes warmer. The problem is then gone and the truck idles and runs perfectly...this problem only appears after the truck has completely cooled down. If I dont let the truck idle at cold start up and I just get in and drive away, I can go 1-2 minutes down the road before the problem shows up. If i let the rpm fall below 2000, then it starts acting like it wants to die and will start "chugging" for lack of a better term. Once the temp needle moves off the 130 mark, the problem is gone and won't show back up until the truck has sat for an hour or two.

I'm a little bit perplexed by this one, especially since i replaced the TPS and IAC and the problem is still there...and there's no trouble codes!! grrr....anybody have any ideas?

 

Unplug the pre-cat O2 sensor when it's cold, and see if the problem is still present.

 

Got around to trying this out on Saturday...unplugged the upstream O2 sensor and sure enough, the problem is gone....I had a new O2 in my parts box already so I decided to go ahead and change it even tho it was already pretty late in the day. Of course the damn thing wouldnt come out....even after several heating/soaking cycles with pb blaster. I even broke off a chunk of the hex with a pipe wrench trying to get it out. Once that happened I said to hell with it and just ordered a new direct-fit cat converter (the bolts at the manifold end are all in surprisingly good shape)

Has anyone tried the direct-fit stainless steel hi-flow cat from flowmaster (P/N 2030005)? Thats what I ordered, it had good reviews on autoanything.com and for $180 bucks I figure it's worth a shot...i'll let yall know how that goes

 

I wasnt even aware that flowmaster made cats. Let use know how it goes. Glad you found your problem.

My collector bolts look terrible, just like my exhaust manifold bolts lol

 

I'll upload some pics of the collector bolts if I remember...I really couldn't believe how good they looked. My truck only has 93k on it so maybe that has something to do with it. Here's the flowmaster cat I ordered:

http://www.autoanything.com/exhausts...8A3838558.aspx