After about 4 mins of operation the engine starts to surge between 1800 - 2200 RPM. It does not ever go back to it's normal idle of 800 RPM. When I remove one of the hoses that runs from the left hand valve cover to the intake plenum the surging stops, but the idle is still at 2000. Any ideas?

Check the idle control valve try to clean it good before replacing.

I have the same problem on my 89 929. When i first start the car it idles really high. After about a minute or 2 the idle starts to serge going up and down non stop. I went thru 3 idle valves. Any other solutions?