idle control valve issue help i have a st162 wth 3rd gen 3sge

Hi folks i am new here & would like to say Hi as i have heard good things about this site

anyway i have a 1986 st162 gtr wth a 3rd gen 3sge from a st202 did full build myself ground up repositioned radiator ect
so on to my issue when i start the car when cold it takes about 7min to settle down to idle as the engine warms up also when car is warm & switched off then started again it takes about 3-4 min to get back to idle (it sits on about 1700rpm then makes its way down to 850rpm in stages of bout 150-200rpm over the 3-7 min time ) i think ICV is problem but not sure can anyone help thanks muchly & if i can assist with any 3rd gen conversions then ask away as i know all the issues involved