SQL Server 2008 R2 memory issue
Although you limited the memory to 28 GB out of 32 GB, this is not the only memory that SQL Server uses.  The running programs and needs within Windows take some additional space.

If you want to closely tune the memory use, I recommend reading this, which works well for my servers.  See Jonathan Kehayias at:
http://www.sqlskills.com/blogs/jonathan/how-much-memory-does-my-sql-server-actually-need/

It basically says "reserve 1 GB of RAM for the OS, 1 GB for each 4 GB of RAM installed from 4–16 GB, and then 1 GB for every 8 GB RAM installed above 16 GB RAM."  This suggests that your Maximum Memory should be tuned to 25 GB.

You can dynamically alter the min/max by setting a new value for those measures.  The OS will adjust in a relatively short time without a reboot.

EDIT: Of course, as both Jonathan and Shanky mention, you still need to monitor your performance counters, since any general answer will likely need tuning.