Slipping window adjustment

[Targa Time]

About every one to two years the window upper limit adjustment slips allowing the window to raise 3mm too high. Over the years I have had to readjust these limits multiple times. It seems no matter how much I tighten the securing bolt, over time it will slip allowing the window to move out of adjustment. I am hoping that from the wealth of knowledge frequenting this site someone has come up with a solution.