When I google Ford Escort Shift Problems numerous topics come up on the FEOA (Ford Escort Owners) site for you to explore. One follows up with detailed instructions to check ALL of the electrical connections. Other ideas include the speed sensor, throttle position sensor, and a cable that controls tranny fluid pressure.
Hopefully it is something simple tied to the service you just performed (a bump or loosening of something). The stuck/lost O-ring. It happens all the time.
I don't know about the Sea Foam, but LubeGard Red get's the o.k. at BITOG and some report miracle cures.
Let us know the final outcome.