Having trouble controlling the speed of a single phase AC motor

I'm trying to control the speed of a single phase AC motor with a variable transformer, but I'm having some problems.

Description of setup:

Motor used:
http://www.blowerwheel.com/blower-gp-3.htm

Variable transformer:
http://www.aaaim.com/u/web/aaaimc/cgi-loca...VATS.htm#VT2502

The motor has a start capacitor of 25 microfarads. There does not appear to be a centrifugal switch. (I can send photos of the disassembled motor if anyone has questions as to its type.)


Problem:

Motor speed control with the variable transformer is very touchy...a change of the transformer's voltage output of just a couple volts changes the motor speed from very slow to near maximum. It is impossible to set the motor speed at a medium speed. I can set speed at a very slow value, or near maximum, but the middle range (which is most desirable to me) is 'unstable' and very sensitive. In many cases after I set the motor speed, it will gradually ramp up in speed over the course of a minute or two.


Questions:

1. What can I do to make the middle speed range easily controlled? Is it possible to do this with the variable transformer? (Or do I need another method of speed control.)

2. If I change the value of the start capacitor...can I optimize the motor's performance and/or speed control characteristics for the desirable middle speed range?

Thanks in advance!