[EC900]

Had similar situation at highway speed during downshift. Turned out to be the two old yellowed 45 yr old nylon bushings for accelerator bell crank in the pedal cluster disintegrated. It threw off the entire geometry of the throttle rod and hung up in the tunnel where three more bushing were also aged out and gone, t-rod was flopping around.
So that’s first to check. Easy to replace if it’s just the nylon bushings (and not pedal cluster rebuild) with driver seat out of the way. For the 3 tunnel bushes passengers seat removal makes getting and seeing these fitted.
Those Bell crank bushes can be replaced easily, nylon or brass but if they were worn down and gone a long time the rod could damage bush opening by enlarging them and will then will need to be replaced.