stuck accelerator

my 2001 4WD Yukon has had accelerator problems since it was new: When I take my foot off the gas pedel, the truck continues at the same rate of speed. I also notice that when the accelerator is sticking, I hear a "winding down" sound or sort of a whine when I break. The dealer says that "they can't duplicate the problem," or, the throttle assembly needs cleaning --- they even replaced the throttle assembly once. Now they are telling me that the throttle assy is again dirty and that the computer needs an upgrade. Has anyone else had this problem and how often should a throttle assembly need cleaning? I've never had a car that required this type of maintenance. Thanks in advance