Something inside the foot controller is out of adjustment, or broken. They are easy to take apart and they are easy to replace. If you do a lot of slow sewing an electronic foot controller might serve you better than one of the resistance types. Sew-Classic has both if you choose to replace it.
Take the controller apart, clean it and see if you can see something out of place. Forum member ArcaicArcane has a blog article about adjusting and cleaning foot controllers. I've lost the link to it so you might PM her.
Joe