When I play a tv episode or movie, very often it starts this stop-and-start thing. Not every time, but often enough to be annoying. Sometimes leaving the site and coming back helps, but not always. Sometimes it's fine for a whole day, then another time I can watch for an hour and then the problem starts. Any ideas about this, please?

Sounds to me as though you computer does not have the capacity

to recieve the show faster than it displays it you.

So it stops playing and refills its' buffer and starts again.

It may be that your 'working area' is cluttered from time to time.

The next time it happens close(not minimise) all windows including browser(s). Then restart your computer. Then play
again. I do not say this will help you but it does help me when I have been flitting backwards and forwards through sites searching for images etc., and then my responses from Sharetv slow down considerably or require more than one keystroke to actually do the job.

How much ram do you have?
What operating system.
How big are your HD's

A couple of suggestions that may help.
1) Exit everything and degrag your drives.
(I like to use Perfectdisk 2008)

2) After defraging do a chkdsk /r for your C:drive.
(You can access this by going to My Computer then right clicking on the drive. Then click properties then tools/error checking. Check both boxes and start. it will normaly tell you it can not be done at this time do you want to doit nect time you restart, answer yes. Then restart your PC. This can take up to a couple of hours.)