2002 Buick Lesabre throttle body

In the past few months I have periodically experienced unintended acceleration while driving along at rural road speeds. I have goosed the pedal and that has stopped it. However this past week the symptoms have become more pronounced. The car starts well and runs well with the fast idle kicking off normally. However, when I go to accelerate the car will maintain that speed reached and will not die off by removing my foot from the accelerator. And in fact the car will often continue to accelerate itself and reach perhaps 60mph. I have shifted to neutral and the rpms have fallen off, but not every time. The rpms have reached 4-5000 in neural and I have shut it off. This becomes more pronounced as the engine gets hotter. The throttle springs and mechanism seem to move freely when cold and pop back easily to idle position, but they are stiff when hot and reluctant to move back to idle. A local shop cleaned the air breather system and lubricated the throttle springs and mechanism but it has not helped. Some suggestion has been made to me that the throttle body needs a thorough cleaning. The linkage to the accelerator peddle seems sound and not binding. Any suggestions as to what to have checked out?