Just an update to this post, today I was getting a 'Sys' undefined error from the AJAX library.
Now this should be as simple as adding a ScriptManager to the page, ensuring the AJAX library is setup in the web.config. So I've spent half a day googling what this error is, and what causes it. However, still no answer to this problem which is now eating into my development time.
I can't work out what the problem is, but if I search for this problem within this forum, guess how many posts come up? 15962! So 15962 posts for this error have been raised by people, and doesn't even include those who didn't ask on the forum.
If I was head of the ASP.NET/AJAX team, I'd be getting all the top problems raised in these forums and then go fix them!! ASP.NET/AJAX is flakey, there I've said it, I've had my rant!