Newbie here...need infor on Jetta idle problems

Just bought a 89 Jetta for my son's first car. Cannot seem to idle properly. Thought it was the idle stablization valve and put in another used one but problem still remains. I suppose both the original and used ISV's could be bad but is there anything else that causes the idle to go from 200-2000 rpm's? Also, at times, the idle is fine.
Thanks for any help you can provide.

I can't remember off the top of my head if that model has the small closed throttle switch on the outside of the throttle valve. If it does check if it's making/breaking contact.