I've nursed an FMX from a Mustang into my '67 F100 to replace the FMX that chewed up a load of roller bearings. The transmission is doing okay, it is sluggish in shifting, but it has a weird habit of not staying in park. The shift arm on the side of the tranny can be moved to park, it may hold for a second, but then it pops into the reverse position. Any idea why it might be doing that? The shift arm does it whether the linkage from the steering column is connected or not. I gotta leave the truck in neutral if I leave the engine running. Don't want to be chasing my truck down the road.