I have a 2000 S-Type with 165k mi. I've had it for about 4 years and got it at around 30k mi and do most of my own maintenance. It had a transmission rebuild at around 60k mi. I'm having a new issue when starting cold. The transmission does not immediately engage, and then drops back out of gear (like it's in neutral) when stopped at a light. Revving it a few times makes it drop back into gear. I haven't noticed any issues with slipping or gear changes as long as you are on the road. My first thought is low fluid, but I'm not aware of a way to check levels or add fluid on this one (5r55n ford sealed 5-speed auto). Any ideas from someone who knows something about auto transmissions or info on how to check and add fluid would be greatly appreciated.

Thanks!