This is a common problem. At the exit of the throttle cable guide is a rubber boot intended to keep out dust and water while flexing with the cable. After years in the weather the rubber dry rots and becomes brittle. In these cases the boot breaks off and slides around on the cable. Under just the right combination of acceleration, braking, bouncing, turning, the boot slips forward enough to wrap around the throttle half pulley with the cable. The cable can't drop between the guides of the half pulley and instead rides on top holding the throttle open to 3000 or 4000 rpm. The fix is easy. Rip that sucker off of there and it doesn't happen again