After selecting 'new posts' I often get only 3/4 through the pages before
I get and error about pages not found.
Sometimes selecting 'new posts' again returns the same list.
Sometimes it shows me only newposts since I logged in.
This makes it very difficult to keep current.

Easy workaround : click through all the new posts pages till you get to the last one. Read what you want, then go back to the previous one using Escape, and so on until you end up at the first page. Then you won't miss any posts.

I quickly work through the list clicking on interesting topics using my middle mouse button which, in Firefox, opens a new tab. When I get to the end, I just read through each tab.

There's obviously a possible race condition, but I can live with that.