The cruise control amplifier (also known as cruise amp or amp or cruise control computer) is under the kick panel. They are known to go bad. Tapping the brake pedal should make it go completely off and if it does not, sounds like a SAFETY issue to me. Most often when the cruise amp goes bad it will cause surging or just cut out.
The other parts of the system are the cruise control stalk on the steering column, and the cruise control actuator in the engine bay. The actuator moves the throttle cable based on signals from the computer.
The dealers have a cruise control tester tool. It tests all operations of the cruise control EXCEPT the computer. If the actuator and stalk check out okay, then its assumed the computer is bad.
If your throttle is not responsive, it sounds like the actuator may be hanging up, possibly because of a strangely working computer. I would have it checked sooner than later or stop using the cruise control. Inoperative throttle control is a safety issue, IMHO.