Lighting, Light Fixtures, Ceiling and Exhaust Fans - How do I slow down a 3 speed fan

I am using a 3 speed ceiling fan motor on a project. It needs to run at the LOW setting of 70rpm. It is a new motor, wired through the control box that came with it. It will only run at the HIGH speed setting of around 200 rpm.
I installed another new control box including 3 speed switch, reversing switch, and 2 capacitors, but the problem remains. I took it back to the store, but they couldn't figure it out. I don't know the make of the motor; the only ID is a tag saying "153x16". The wiring from the control box to the stator - through the hollow shaft -- is red/yellow/brown/purple. I need to get it running on 3 speeds, or just running at the LOW speed would work -- if it will start. Any ideas?

I am not an expert on electric but I bet an inline adjustable rheostat would work!

I've tried plugging it into a router speed control, and an electrician friend wired up a variable motor control in a box, but neither would slow it down.