firewall blocks INTERNET if I leave PC for more than 3min.

Grue, noted your comments…

Aka, have you checked if its a problem with your ARP… how many network adapters you have in your system.
to check arp issue go to commad prompt and type

arp -a and strike enter
if you see and network adapter ip with some unknown internet ip and 00-00-00-00-00-00 in your physical address you are having the same problem as i do

when you reboot router this entry will not be present in ARP cache

If you have two cards and you disable/unplug the jack in the other LAN card , does your connection keep working?

My PC is connected through USB port and the wife's is connected to the ethernet port [there is only 1 of each]. This also saves having to buy a hub.
Been there/done that with a router/FW with only 1 ethernet and 1 USB port. Not a single problem on the computer connected to the ethernet connection but always a lot of problems on the USB side. No problems after D/C the USB & purchasing a switch so all the connections are ethernet. Switches are dirt cheap, at least in the States.