Curious if you ever got this idle problem nailed down... I have 95 K2500 that with a 454 (Gen V TBI) that i just rebuilt (everything back to stock except bored over .030"). I've had difficulty controling my idle every since i've owned the vehicle, even before the rebuild; and i've tested/replaced about everything that you've mentioned. I also have inconsistant plug symptoms... Half of them are carboned very badly (some of them have deposits on only one side...), and the other plugs look as though their associated cylinders are running too lean... Its also been running a little hot, and i think im loosing a little coolant (internally.... uh oh!)..
I just finished reading some info at
GM TBI Swap that mentions a park/neutral switch that is supposed to adjust idle when in park/neutral... I was unaware of such a switch, but sounds like it could be related to your problem....
Please let me know what you find/found out
Thanks,
Someguy