I have a 1972 350SL. When the engine is cold, once I start it the rpm will slowly creep up. Seems normal.

However, once it hits about 1700 or so rpm, the rpm will quickly drop to about 600 (in about .75 seconds), then quickly accellerate to about 1700 rpm (in about 2.25 seconds) then repeat. The engine will idle like this until one of two things happens: the engine warms up and the rpm evens out, or I stick it in gear as engine doesn't seem to do it under load.

Once the engine is warm, it never does this, even if I start it 30 minutes after it's been running.

Has anyone seen this? Any advice would be appreciated.

Thanks!