Is the hashrate changing throughout the day normal?

I just setup a card yesterday (usually use it for otherthings but figured when it's idle might as well make some eth). After some tweaking I got it to generate around 57 MH/s which I'm okay with but this morning I noticed it around half that. I restarted the miner and computer just to check and it's the same. So, is this normal, what would cause this drop? Should I go back and tweak OC settings to see if something is up?