1999 Suburban Stumbling

I have a 99 Suburban for about 4 years now and it has about 220,000 miles on it and it has been nothing but great! However, recently it has been stumbling when you try to drive it right after starting. Usually, my wife will start it and immediately try to drive and it stumbles for the first minute and then seems to be ok. It is almost as if the engine needs to be warm (even though it has been 100+ outside for the past 3 months). Also, I have notice the odor of gasoline coming from the back of the vehicle. The fuel punp was replaced about a year ago and, come to think of it, the odor started right after that. The stumbling happened at first when we got the truck, but a new fuel filter took care of it. This time, I have replace the fuel filter, plugs, wires, cap, rotor, and air filter and it does not seem to go away. I would really appreciate any and all advice! Thanks in advance!

Another part of this problem is that it has been throwing the code P0420 for quite some time. The severive engine soon light will come on and then after a while (like days or weeks) go off for a couple of days and then reapear. Not sure if this is related.

Look at what I just did.
http://www.automotiveforums.com/vbulletin/showthread.php?t=1038720

What motor's in your Sub?

Look at what I just did.
http://www.automotiveforums.com/vbulletin/showthread.php?t=1038720


I will check the fuel regulator this weekend. This Sub has the 5.7 in it. Thank you for your help!