How many miles on it?
I presume that downshift would be from fifth to third? To go to neutral would be difficult as I think both fourth and fifth are additive. Meaning that you are already in third if you remove fifth and fourth. I will have to look at a powerflow chart for fifth gear. In the 722.3 and 4 trannies 3rd and fourth gear are clutches and are additive. The 722.5 might do it differently.
I would suspect a command valve problem in the valve body. How long since the last trans service? Depending on the age I would recommend a additive designed to soften seals and free valves. Lubeguard is a good product.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician