Code P0507, no one can fix so far

My ride is a 98 blazer, 4.3 with csefi(can replace each poppet valve) and a 4l60e. It started with drooling oil cooler hoses, where I discovered collant on the bell housing, leaky intake. Replaced the intake gaskets, cleaned the lower intake, replaced the plastic plenum, cleaned throttle body and iac port. The water pump, the fan clutch and hoses were also replaced. I took it for a road test and it felt like it had either a lazy iac or a binding throttle cable. Cruise at 70, shift into neutral and it stays @ 2000 rpm. Come to a stop and the rpms are also high and will slowly drop to idle, which in gear is 750 in park 950. Swapped the egr, no affect. Tried a new iac valve, no change. If I unplug the iac, then start it, it runs @2300 rpm. Plug it back in and it drops to 950. It idles smooth. I also changed the cap, rotor(brass tips), Belden wires and a set of ac double plat's. I have unplugged the ac clutch(which is seized anyway) and even "borrowed" a map sensor. I used an entire can of tbi spray hunting for a vaccuum leak. I have even tried harsh language and kicking the tires. I have some info off scanner if it helps much.. THAnks!

Can not help much on that one.
But Here is what the good book says to do.
http://members.troublecodes.net/crunch/507.pdf
let us know how it goes.
Good luck MT

hows about a throttle position sensor?

I did read that the PCM has to re-learn for correct idle speed after the IAC has been diconnected.
see if you can sense vacuum in the crancase(intake leak fromcrankase side)