How is the spring arranged? Does it pull directly opposite of the cable, or is it on the same side as the cable, but on the opposite side of the throttle shaft? I can't imagine you have a single spring that stiff, but even the OE stuff tends to waller the shaft bushing out eventually if the spring is on the same side as the cable. It does not however do it anywhere near as fast as you describe.
Just something to think about.
