I think this book was highlighted on Reading Rainbow.

A boy goes back in time via a candle and cave(?) to King Arthur's time.

Maybe A Connecticut Yankee in King Arthur's Court or something based on it?

http://en.wikipedia.org/wiki/List_of_Rea...

Here is a list of Reading Rainbow episodes and the books talked about in them. Didn't see any that match your description but maybe it will help.

Possibly one of these.
http://www.worldcat.org/search?q=kw%3AKi...

So more about how I learned of the book.

It must have been in the late 60s or early 70s. I watched a tv show about books at school. That's why I thought it was Reading Rainbow. But it looks like that started too late for me.

Thanks everyone who is trying to help.

You know, I'm wondering if this is Earthfasts by William Mayne? The publication era is about right (mid 1960s) and there is a cave, a candle, etc. If I remember right, time is either shifting or breaking apart? Anyway, King Arthur and his Army are sleeping, waiting for a time when England is in need.

Check out the touchstone or on Amazon and see if you think it's what you're looking for.