Could be bad drivers, a game bug, or just plain bad coding. Apparently BF4 under Windows 7 has been having a lot of issues, I guess MS threw a ton of money at DICE to cripple Windows 7 performance in BF4 to try to drive up Windows 8.1 sales, or they simply decided not to bother testing in a Windows 7 environment to save time and/or money. In that case all you can do is hope for a patch or driver update that fixes these problems.