Welcome aboard! Does your machine have the outlet with two plugs in it? It's a common thing for the motor to get plugged into the outlet intended for the light. That will give you the all or nothing results you describe. The other possibility is a bad pedal. They work with a either a coil or carbon pile resistor inside. You basically bypass the resistor at full speed. If your contacts are dirty you might get those results. They're not hard to take apart. You might be able to fix it. If not, new foot pedals are relatively inexpensive. We like Sew-Classic.com for parts. The owner is knowledgeable and the prices are reasonable.
Rodney