Most lenses with helicals have some form of grease in the threads to smooth the operation as well as providing necessary lubrication. The cheaper lenses have coarser helicals with thicker grease that after a few years works out and the focussing gets floppy. I'm sure most of you have seen some of these. With any after years of use there may be minute amounts of dust (or more!) work into the helicals and get imbedded in the grease. This is great considering it goes no further into the lens at that point but will make the action stiffer and sometimes uneven. Also some of the greases used will harden over the years and make for a stiffer action. If you value the lens highly a trip to the repair shop for a good cleaning and relube should return the action to a "new" feel. It won't be cheap so I wouldn't even bother for the mid range to low end 35mm lens. Be cheaper to buy a new one.