[W126 Coupe] Vacuum problem?

[David Fatovic]

I was doing some work on my dads 85 500 SEL last
night, and one of the first things I did was power
wash the engine.  The valve cover gaskets were so old
and hard they were letting oil seap out on to the
exhaust.  So, steam cleaned the engine, then replaced
the valve cover gaskets.  Now (actually had happened a
bit before) under load from about 1400-1900 RPM the
engine spits and sputters, shimmys and shakes.  I have
a feeling it is a vacuum problem but not sure where to
start.  Once you get past about 2K RPM, the engine
seems smoother.

Any ideas?

Also the car in Drive used to idle around 1K-1200 RPM
now it idles at about 650-700 RPM and shakes doing so.

David