89 300SE surges at idle

The idle when started surges from 800 to 1500 then back again. The car has about 150k miles. Runs fine at high speed.
Any suggestions on what could be the cause? Also, I wanted to check the fuel pressure but don't know where to connect. I don't see a test port, other then the 12mm(I think) bolt type port on the front of the fuel distributor head. Can I get a fitting for that? If so where. My fuel pressure gauge has larger threads made for American cars.
Thanks
Jerry

The idle when started surges from 800 to 1500 then back again. The car has about 150k miles. Runs fine at high speed.
Any suggestions on what could be the cause?

I guess no one reads these questions! Can someone suggest a better site for Mercedes?