Sounds to me like it could be a vacuum leak. Are there any other symptoms?
The last time I changed the oil on my 300E I knocked the trans vacuum line off of the intake manifold. It had a rough idle and the transmission shifted harshly. It was a piece of cake to find the problem because shifting was a good hint.
I'd take a look around in the engine bay and see if there's a vacuum line unplugged somewhere.
Experience is something you don't get until just after you need it.