What happened to the "show only new posts" function?
Without this it is a huge pain to revisit threads one has already read. I can't imagine that management wouldn't realize this, so I doubt they have done away with it deliberately.
Is there a malfunction?
This is a bug we've done a fair amount of research on, but have had a very difficult time tracking down. The problem is inconsistent, as you've both noticed- it starts and stops seemingly on its own, only effects certain threads, and is otherwise without rhyme or reason (see http://chowhound.chow.com/topics/828712 for a larger discussion).
If you've noticed any patterns in this bug that might help predict when it occurs, please let us know. Thank you both for your reports.