Dremel is varying in rotation, even if it should hold constant speed.

This proceed worked for another with the same issue:

Roto Variable speed: Repair I carefully removed the 4 torx head screws and the hanging clip and eased the 2 halves apart. Not expecting anything to drop out, I was startled when something inside bounced onto the floor. I quickly found this mystery metal piece and proceeded with my exam. Not seeing anything that appeared to be broken (switchwise), I grabbed a can of WD40 and sprayed the switch and bearing areas. After figuring out where the piece went that had fallen to the floor on disassembly actually it was the metal button used to hold the shaft when changing bits), I carefully assembled the ailing patient, tightened the screws and plugged it in for a test run. To my delight, not only did the unit function as it was supposed to speed control working) but the unit sounded better. Not rough like it had previously. ANOTHER: Did your variable-speed tool stop being variable? If so, the speed control's contacts are probably dirty and oxidized. 

Here's how to fix it. Remove the brushes and carefully take apart the tool's case halves. Remove the motor assembly from the case halves and place the switch in the off position.

Spray out the switch contacts area with electric motor cleaner aerosol. You can also clean the commutator area while you’re at it. Let it dry and then spray some CAIG DeoxIT spray onto the switch contacts area. Re-assemble the tool and hopefully you’re back in business. You can get electric motor spray from any automotive parts store but the best ones are the ones for RC cars from the hobby stores because it is less harsh and cleans just as good. The CAIG DeoxIT can be found in specialty electronic supply stores. DeoxIT