I agree, the fluid must be checked with the car on level ground at operating temperature. The amount of fluid between the two marks may only be a cup or two NOT A QUART. The level can easily raise from the low mark cold to the full mark hot on some trannies.
When the trans has the fluid level up to the height of the rotating drums the fluid foams and can come out everywhere including the breather (top/forward/center). The foam also is compressable so the transmission devices (band servos and clutch pistons) aren't applied properly. This can result in loosing top gear all the way to no drive at all.
After the foam goes away the tranny usually works again, unless you really beat it while its dying.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician