Raising/Lowering
The above posts are right on and the most likely causes of " catching " I just went thru a complete cylinder removal, replacement and repair. I went over the entire system. Here is a summary of what I learned.
1. Guide Rod: As the cylinder goes up and down it is supposed to make the pulley head slide "smoothly" on the guide rod. If the guide rod is dirty, corroded, bent etc. it will cause catching and chattering. My rod was bent 1/2" out of line and completely jammed the system. Clean, grease and inspect.
2. Pulleys: All pulleys should rotate as the cables move over them. Any frozen or stuck pulleys could cause catching.
3. Cables" Make sure the cables have no broken strands or "fish-hooks" that could cause catching.
4. Guide rails and blocks: Get a flash light and look up at the guide blocks that align the top on the rails. Make sure nothing is loose.
5. Top Level? Make sure the Top is the same measurment up off the supports. It could cause binding if it is cock-eyed.
6. Fluid level: With the top down, make sure the fluid level is OK in the reservoir. 1" from the top. I know, it is tough to check with the top down but it can be done.
7. Air in the system ( Long Shot): The system seem to be self-bleeding and should purge out any air after a few cycles. But air in the hydraulics could cause "chattering"
If you can, get under the unit and observe all components during raising and lowering. Best as a Two man job but I did it one-man using a auto remote start switch jumped from the battery to solenoid. You will probably spot something.
Good Luck !