PO507 iat rpm higher than expected

[gagliano7]

I put a 5.3 in my 1992 pickup 2wd 5 speed with a stand alone harness cable driven throttlebody.Every time i start it cold or hot it will idle up to 1500 rpm back down to 700 back up to 1500 then back down and idle fine.i put a new throttlebody and sensors with no change.I sent harness and computer back so they could test on there motor and they are having the same problem even when they changed computer and another wiring harness.I think it is in there wiring harness both built by the same person but i would like to get your opinions.

Thanks Joe