I may have stumbled across the cause of your stumble problem myself.
Take a look at the linkage which connects your front and rear secondary butterfly valves. This will be under the Upper Intake Manifold (UIM) just in from the manifold end nearest to the throttle body - it's round wire about the thickness of a coat hanger.
Make sure that that linkage is connected on both ends!
There is a little plastic retainer (at least on the front bank-side) that broke on mine - this disconnected the front butterfly valves. It may have happened with yours too.
An interesting thought on this...could it be that if the butterflys don't open and the ECU blasts extra fuel anyway, it would throw a HEGO failure code?
I expect it would because I didn't find a sensor on the front valve-shaft and the ECU would know that the fuel wasn't burned completely (this is measured by the HEGO) and infer the sensor was bad because the other bank was fueled the same way with a good HEGO signal.
Basically, the ECU would know something was wrong but miss-diagnoses the root-cause.
Perhaps the moderator could chime in on this idea. Frankly, a sticky about the missing retainer and what it can cause would be a good idea.
For sure the missing plastic retainer would allow the secondary valves to do something other than what they should.
