Joanne,
The foot controller is a simple two wire resistance unit. There are both resistance and electronic controllers available for it.
However before you get to the controller let me ask this. When was the last time the machine was thoroughly cleaned and oiled? And make sure the motor belt is not too tight or loose.
As for the controller here is a couple blog entries about them:
Drat, those are two links I lost in the last computer crash. They were from ArchaicArcane's blog.
If you PM her I'm sure she can give you the links, or someone else who reads this might have them.
Also if the controller is the button type, you can disassemble it and clean the insides, making sure there is no corrosion or broken pieces in there. That also might help.
JArrrghhhhh, I'm sorry I couldn't be more help.
Joe