I have a 1992 Ford Tempo GL automatic (4 Cyl/2.3L) that has started
intermittently giving me problems when it tries to automatically shift
between lower gears.  I have to really press down on the accelerator (and
it feels really stiff) to get it to shift up.  The engine revs quite a
bit, but it will always "jerk" into the next gear.  Also I notice a
similar problem as I ease off the accelerator.  As the engine slows, the
car will abruptly shift down into the lower gear, which is also "jerky".  
All these problems occur between 0-60 km/h, there are no problems
whatsoever at higher speeds.

I have done the following:

(1) Cleaned the throttle vale with carb cleaner and a toothbrush.
(2) Replaced the air filter, spark plugs.
(3) Checked levels of transmission fluid, oil, etc.
(4) Replaced the EGR valve
(5) Inspected the throttle linkage cable

The problem still persist.  It usually does this once in every 15 minutes
of city driving.  I've never had a problem (knock on wood) on highway
driving.  This happens regardless of the outside temperature, or whether
I'm driving on a full tank of gasoline, or an empty one.

Any suggestions would be greatly appreciated.

Thanks.