904 misbehavin'

I guess too week of a spring coupled with a build-up of grease or other drag could cause the problem you've described.

But it's important that the spring be weak enough so that it is overcome by the throttle return spring. If you pull the kickdown linkage forward the throttle should stay closed. If the spring it too strong it will overcome the throttle return srping and open the throttle. That's what the safety feature was designed to prevent.

I guess as a test, you could put a really strong spring on it or even a peace of wire. In that case you would be at no more risk than if you were driving the early design without the slot. If the problem goes away then you can try to locate a spring with the proper strength.