It's too bad Ford didn't equip the 5R55S with a dipstick 'cause my first instinct, from the symptoms you've described, is that the fluid level is low. There could be a leak and the most common source is the pan gasket.
I suggest you drain the pan, remove it, and change the filter as well as the pan gasket and fluid. Those simple measures might solve your problem. While you have the car on a lift, check if there are any other fluid leaks esp. output shaft housing gasket.
If none of that fixes it, there's a deeper underlying problem that's keeping the line pressure low e.g. valve body interface leak, oil pump (less likely).
