I had a similar issue recently. I had a 1997 Taurus that would appear to go into neutral as I slowed or stopped and would suddenly re-engage as I attempted to accelerate. It would sometimes fail to go into any gear until I shut the car off and let it sit for a few minutes. My self diagnosis after removing the valve body is that I suffered from a cracked forward clutch piston. This is a common failure requiring transmission rebuild. I called someone and had them take the car away. I hope you are more fortunate. | 
