Campagnolo downtube friction shifters were known to be prone to slipping. Ironically, it might have be a consequence of their manufacturing their components to very high standards of precision.

If the problem recurs after you tighten the D ring, one fix is to degrease the threads of the D-ring bolt and then add a drop of blue Loctite to the threads.

Or there's the quick-and-dirty technique I used with my Campy down tube shifters back in the day: I simply grabbed a Vice Grip and crushed the D-ring threads ever so slightly. Works perfectly and appalls the purists as a bonus.