I've had about the same thing happen to me flying a C-172SP over
western Vigrinia at 10000, the foothills east of the Appalachians.
Autopilot was on, level, smooth as can be, and within a minute or two I
notice the RPM indicator dropping by about 150RPM, and airspeed going
from about 110 IAS to near 80. Altimeter and VSI both reported level,
and I couldn't detect much of a climb on the attitude indicator either.
Throttle was full in as well. I chalked it up to a downdraft.

--
Guy