The usual cause for steering problems such as you described are worn or broken bearings or a bent shaft. Cleaning or lubrication obviously won't fix either and the only way to tell is to remove and inspect them.
A friend with the same boat and same problem, dropped the rudder, cleaned it and since then (2 years ago) is really smooth. Worth trying the easy way first, but cannot drop the rudder now.
I sometimes spray wd40 on top of the rudder shaft (a really small area above the bearing that you can actually see the shaft), with the hope that some will get between the shaft and the rudder and clean it a little bit. The I spray a lit of bit of Mclube one drop. It does make a difference but doesn't last for long. I need to find a cleaning "oil" or something that could dissolve any dirt accummulated there but not damage the bearing or the shaft.
(I once tried silicone spray and was a disaster. The helm almost got stuck for some hours!).
PS: The bearing is nylon (Ertacetal)
PS2: Probably I need some kind of cleaner/degreaser that will not damage GRP or the bearing. Could "isopropyl alcohol" be the answer?
