For some reason, within a couple of minutes of loading up the site it marks all posts, forums etc as "no new posts" which means manually trawling through them!

You're not logged in - so it doesn't know who you are and when you last visited.
Happens sometimes - dunno why
Just Login .

Nope, definitely logged in, first thing I checked! It's done it again this morning which is a bit irksome.

still doing this - I get about 20 minutes of being logged in to read everything / anything new before everything is marked as read.

It been the way it's always worked for me. Not found it a problem myself.