It sounds to me like you are low on fluid---but
DO NOT mix fluid types.
I am not sure on the exact science of all this, but you can use ATF. HOWEVER, you cannot mix it with the hydraulic fluid that is in there. They somehow react with each other or something and gum up the lines / pump.
I had a motor that would stop every now and then and not move at all. I had it rebuilt and when I reinstalled it, the shop told me to add some ATF to top off the fluid level once it was connected. It worked for like 3 days then stopped again. I brought it back to the shop, they bench tested it and said it was fine. I ended up replacing the motor and the lines (one of the biggest PITA to install without removing the cylinders). I filled all the new stuff with ATF and had no problems.
The point is, I am not sure if adding ATF ruined the first motor. I would make sure that you're adding the same fluid that is in there or drain the pump reservoir and the lines and then refill the whole thing with new fluid. If you do that, you will need to bleed the lines of air before reinstalling the whole thing. All in all it took me a good part of a whole day to get this straightened out.
Good luck.