Where are the restrictors located in the circuit?

On the pneumatics we use at work, most of the restrictors are 1-way to avoid this problem, only acting to restrict flow out of the cylinder. If you're feeding pressurized air into a cylinder, you can limit is speed on either end. Problem is, when the cylinder gets 'sticky', an inlet-side restrictor can cause the pressure to build up, then overcome the stiction, then the supply of pressure is not enough again and it stops part way through the stroke. The pressure then builds up again, breaking the piston loose again and the cycle repeats itself until it hits the end of stroke...

If the restrictors were placed on the outlet side of the circuit, the disadvantage is that the cycle will begin rather fast, then slow down once the outlet pressure builds up behind the outlet orifice. It should come to a comfortable medium and flow evenly from there right to the end of stroke though. I think this is how the Robart valves work although I'm not sure of it... I think Yellow Aircraft the retracts in my Stingray are the same.

Regardless, I think your best bet is to start with relubricating the cylinders and joints. I did this with a syringe and air tool oil. You won't need much... Grease may be too thick for the application. Remove the retract from the model and cycle it each way a few times after putting a drop of oil on the cylinder rod to make sure the seal is wet. (first wipe off any dirt of coarse) Next cycle the cylinder a few times each way, adding a few drops of air tool oil in each nipple while its on the suction stroke. You should feel it loosen up noticeably if that was the problem. Next keep cycling it with a rag near the output to catch the excess oil, making sure the nipple is at the lowest point so it won't pool inside the cylinder. Keep doing this till the excess oil stops flowing. Wipe off the nipple with alcohol and reinstall the hose with a wire tie or one of those Robart retainer rings and it should be good to go.

Hope this helps.

Kelly