"The last few days when I've tried, they keep stopping and starting - very frustrating - so that I tend to give up before even half a film is watched."

The rate of data thruput to you is now much slower, either

because your ISP has found that you have reached your data limit for tehmonth and has switched you to a slower speed (called shaping) for teh rest of the month.

or YouTube is now reaching the capacity of its servers, due to popularity.