This is a sign of a fault TPS (throttle position sensor). When this occurs, replace the sensor even though it may pass tests.
An easy, five minute, $40 fix with only one small "trick" to installation. Where the sensor slides onto the "tab" on the TB, it must be "preloaded" the correct way. Simply cock it clockwise about 20 degrees, slide it onto the "tab" all the way, then turn it back counter clockwise until the screw holes line up. Bolt it on and enjoy your cruise control working!