I was googling this over my lunch break today. This may not all be correct, but it is the gist of what I found. To stay logged in, the forum functions by having a cookie on your computer or device with a 1 week refresh period. Every time you log in or access the forum it gets refreshed for another week. So as long as you're here at least once a week, it should keep you logged in.
The forum annoying will log you out of one device if you try to log in with another however. This is so there is only one active session, but it is more annoying than helpful. This is where the issue lies. When you get a new internet provider, you also get a new set of IP addresses. It takes a little while for the forum to build up a repository of IP address and device correlations. Until it "figures out the pattern" it is confused into thinking you're logging in from a new device every time. It goes away eventually. This happens to everyone, even me, and there isn't a lot I am capable of doing about it other than apologizing and telling you to keep with it.
As far as the pm thing could you check something for me? When you go into the messenger thing (http://www.amateurpy...odule=messaging) there should be a bar on the left side that says "Storage". There's a limit to the number of private messages a member can have in their inbox at any particular time. Just make sure that isn't maxed out. If it's not I'll snoop around, but just wanted to check the easy solution first.