Did you check the throttle return springs? There should be three of them, IIRC. Sometimes they wear out or snap. I had only two on my '69 at one point and one of them snapped while driving. The remaining spring was not sufficient to get the throttle to come back, so the car started running away from me. Fortunately, I realized I had a stuck throttle and was able to kick the clutch and kill the motor before I hit anything.
The other thing you can do is disconnect the springs from the linkage and see if the linkage itself is binding anywhere. It could be something in the linkage is bent or might just need lubrication.