Hi!!
I've been getting the same P0505 code. I already got the IAC valve replaced, couple of days later I got the same symptoms back.. Idle gets stuck on around1500 while in nutreal, while it's in firtst, or second gear it'd go up and down, like I was playing with the gas pedal. One day it'd do it, the next day it won't.
All vacuum hoses seem to be fine..
I ran out of ideas..

Tom

 

Check that the throttle plate and cable are not sticking. If the throttle is stuck open a little so the ECU can't get the idle down, it may throw the IAC code.