Hi! Is the steering stiff even if you use sidesift or tilt to the end, and steer at same time. I had once similar issue and the fault was stuck steerin relief valve. Did you mean drivespeed is slowing down at same time? Then it cant be mechanical broplem in hydraulics.
I didnt hear about cpd205? I guess its cpd20 or 25? Is it basic four wheel counterbalance with AC motors and what is the year? I guess you will find zapi manuals on its website. Heli manuals are poor.