Hmmmm...this might be worth looking at. A new code has appeared tonight - P0505 and it kept stalling (wouldn't idle). Either this is related to my current problem or I've got something else to deal with now.
The reason I suggested it is because I too have been chasing an intermittent 171. When I got the vehicle, it was sound as a pound. Then it would throw a 171 code and idle like crap and stall at lights when warm. The exhaust stunk to high heaven.
I played the "What the hell do these voltage readings mean" game, and got no where. A post to

here came back and said "replace the O2 Sensor".
I took a trip to the JY with the intention of replacing every sensor on the TB and manifold, when I found I could not find any IAC actuators, few TPS sensors, but all the MAP sensors were still on the vehicles. Hmmm, a trend?
So I took mine off... and it was broken. The stepper motor and plastic guides are not made of much, and after 190K they gave out. Sometimes the IAC would adjust, but most often it would mess up. So I replaced it. Now it idles normally at all times. The 171 code went away around town.
Presently, I can drive the vehicle for hours without a problem, and unless I crack it open (WOT), the code stays away. When it does occur it is on a very long drive or some decent WOT operation. The code preceding the 171 is a 131, which indicates slow O2 on bank 1 (manifold). The engine runs normally and the exhaust doesn't stink, it just throws a code which goes away after a few starts. I plan on replacing the O2 Sensor when it gets a bit warmer.