Mysterious Tap Action With Sink Combi Tap

[surd100]

I lie awake at night fretting over a problem with the action of my sink's combi cold tap. When I open it, the flow stays very low and after another turn or so, the tap feels as though it's binding. Woggling it back and forth a few times eventually frees it, restoring normal action. Oddly, it can also be freed by woggling it before the point where it seems to bind, although the tap's action sometimes then reverse (opening reduces the flow and vice versa). If I unscrew the tap then reassemble it, normal action is always restored. I have also wire-wool cleaned the tap's inside bore and re-greased the thread. Yet within a day or two the problem always returns. I can't afford to hire an industrial X-ray machine to get to the bottom of this problem. I'd be very thankful for your suggestions, particularly from anyone who has encountered a similar problem.