High idle issue

i'll try to keep this short

tested for vacuum leaks, IAC valve is fine, fast idle valve is new

i properly adjusted the idle (car full warm, unplugged IAC, screwed the screw in, plugged IAC in, reset ECU, turned car on, idle for 10 mins) and with the idle adjuster screw alllll the way in the car idles at like 1250rpm.

with the screw farther out it idles at 2100rpm. with the screw as-is the car idles at 1700rpm when first turned on for about 4 seconds, drops to 1200, then moves up and down between 1100 and 1500. if i retard the timing pretty severely then it wont move up and down, otherwise it will.

TPS sensor? MAP sensor? something else?

Have you checked the coolant level? I know that might sound weird but the Fast Idle valve uses coolant to determine the temp of the motor. If the coolant is low and air is being passed through the motor then the fast idle thermo valve will read a cool temp and cause the idle to change.