SQL 2008 R2 - Memory Min and Memory max settings

Using Windows Server 2008 R2 with 8 GB Memory, and SQL server 2008 R2 SP1

The Memory Min and Memory Max are both set to 5 GB.

After running for a few months SQL uses 7 GB of memory, and we start getting SQL timeout errors as well as other strange errors.   If we reboot the server, all is well for a few months.  Then it happens again.

Any suggestions?

Thanks

Tom

MisterT99