Gnus hangs when not used for 5 minutes

Hello,

I'm using Gnus for 2 weeks now. It's a great program but there is one
thing that makes it almost unusable.

My workflow:
- run Gnus with `M-x Gnus`.
- Gnus creates list on unread messages.
- I walk away for 5-10 minutes to do different tasks.
- Whe I return to Gnus I see that any attempt to view next unread
  message hangs program.
- Gnus can be `unhang` by pressing `C-g` but will not be able to
  do anything with internet connection.
- So I exit gnus by hitting `q`.
- I run once again `M-x Gnus` and everything start over again. Gnus can
  connect to servers but only if there is no break when using it.

To me it looks like Gnus needs some kind of "keep alive" or "heartbeat"
mechanism.

Is this known behavior? Maybe I should configure something?
Can Gnus download all unread messages to my hard drive so i could view
and respond to them in offline mode?