I had a factory rebuilt 2.1L installed, replacing my perfectly running
1.9L about 8000 miles ago.  The old engine had developed low oil
pressure after 165000 miles.  The new engine has run generally OK except
for  a "bucking" condition when coasting while in-gear.  This happens at
about 1200-1500 rpm no matter which gear I am in.  Another glitch
started about 2 weeks ago (about 2 weeks after a $400 tune-up at the
dealer).

When the engine is cold, it idles very rough-say 500-800 rpm- and
invariably dies unless I blip the throttle. When the engine warms to
normal operating temperature, the idle unsteadily oscillates between
about 1000-1300 rpm.  Along about this time, fuel mileage dropped from
about 18+ mpg to about 16 mpg.

Anyone have any suggestions?

Mark