strange bug on router with bridged interface

We discovered a stange router behaviour here… We have network with public IPs in it, router is connected to it using bridged interface. After we upgraded to version 2.9.9 we encountered a problem. When we start using new IP in that network we get good speed on local connections but extremely slow Internet. (I really mean extremely - 1 kbit/sec for example) Upgdading to 2.9.10 didn’t solve anything, same as downgrading to 2.9.8. Couple of hours of testing and we found very strange solution - this glitch does not happen if we create new line in firewall (for this new client) not making new line directly but copying existing working one. Did anyone encounter such a problems? Is there a reason for so strange behavior?