I had the same problem with my Bendpak two-post lift doing the same thing. Turned out the hydraulic hose that goes down the inside of the post was turning so that the elbow was wedging itself between the base and the arm. The little tab that was holding the hose broke off so I had to make...
I have two Bendpak XPR-10A in my shop and I'm very happy with them. I did a lot of researching and even talked to the reps at Rotary and Bendpak about their products. I chose Bendpak because I wanted safety over price. Here's a caparison link I found very helpful...