When I first got my Chevy I thought the transmission was slipping when cold. By checking the RPM I saw that they were a lot lower than I thought. It turned out that the fan clutch was engaged for the first few seconds after starting the engine and the noise of the fan gave me the impression than the engine was revving way higher than it actually was. I finally found the explanation in the user manual that this was normal.
I know that Sprinters (some at least) have a viscous fan clutch. Are the RPMs really as high as they sound, or could it just be noisier than usual? Do you have an RPM meter to check?
William