Idle speed on Ford KN Laser 1999 Auto switches between 1500 and 900 RPM

Ever since I’ve had my 99 Ford KN Laser Auto, I’ve been plagued with idling issues. When I first got it, it was barely idling at about 500 RPM. I’ve replaced the plugs, TPS and crank sensor after it shorted out. Now it usually sits around 8-900, except when it doesn’t.

At cold it idles at 1500 and doesn’t come back down once the engine warms up. However if I cut the motor and restart it, the idle drops back to 8-900. Likewise when driving, sometimes I’ll notice the engine running on when my foot is off the accelerator, when I drop it into neutral it’s idling at 1500. If I cut the engine and restart it, it returns to back to 8-900.

Any idea what might be wrong with it?