With so little info to go on, I'd say likely a bearing. While you are checking on them, check the rubber seals the run against the inside of the screen down on the bottom to create the dead space so the screen can clean. There is also a brush seal that runs in a channel on the outside diamter of the screen, it may be needing cleaning, but not likely the sole cause of your problem.
Those bearings are a ball bearing type that has the bearing balls running in a "cage" to keep them seperated while spinning around in the races. Sometimes the cage will fail and the pieces will jam the bearing balls or if it is a plastic cage, it can melt and allow the bearing balls to run against each other which will cause the bearing to stall or freeze up till it allmost stops, then allow it to go again for a bit. This will eventually stop all together when the bearing finally seizes for good.
Early (98-99) 2300's rotary air screen shaft would deflect enough between the two bearings to vibrate and cause the screen to stop. The shaft had to be made heavier to prevent this due to the increased rpm of the screen on 2300's. The rad fan is different than previous models and moves more air. The part number in the system for the shaft is the new style, make sure the bearings are good, hardware is tight and that none of the support brackets or fan blades are broke or cracked as a result of the vibration.
I agree with what redhat said they updated the shaft I believe from a 2 hole to a 6 hole (where the screen bolts to the shaft) we had the same problem last year on ours and replace everything untill we heard that they had an updated shaft after we put that in ...no more problems.