After 3 years of good service, the plug in the pop-up waste in a bathroom basin won't stay down/closed for more than 30 seconds. It takes me 2 minutes to shave, and in that time the plug pops up 4 or 5 times.
While it is closed the basin is water-tight and when it pops it drains at about the right speed, so I don't think it is the height of the adjuster under the plug at fault. It seems as though the weight on of the ball on the end of the control is pushing it down, but until 2 weeks ago this didn't happen.
What should I try adjusting to make it work properly again?