Mondo Technology has a PIC microcontroller circuit for running your Dremel at slower speeds.
“The Dremel drill fitted with a mini drill-chuck on a stand is a great setup for drilling circuit boards. The problem is that it does not do low speeds very well. My unit had a failed internal speed controller, so I just wired around it and connected it to a regulated 24 volt DC supply. This worked OK, but I would like a slower speed when drilling boards. Slowing down with a lower voltage, however, causes it to stall out completly when it begins drilling. What I needed was a circuit to actually increase power to the motor under increasing load.”
Speed Stabilizer for Dremel Drill
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You may want to try using a Variac.
they work grate when you need a smooth voltage adjustments.
http://en.wikipedia.org/wiki/Variac
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A simple PWM controller would be your best bet. You could even build a simple one with a timer chip driving a transistor with a potentiometer to adjust the frequency of the timer.
See this: http://www.scienceprog.com/control-motor-pwm-schematic/
And more options here: http://www.discovercircuits.com/P/pwm3.htm