MacBookPro Becomes slow

I have an early 2011 MacBook Pro with an Intel Core i7 processor @ 2 ghz 16 Gb of RAM. I just changed the original internal HD to an SSD. It really flys. The problem is that when I leave it on for more than an hour or so, the computer becomes very slow. If I restart it the problem is solved, but sometimes I cant do that because I am doing a big backup or compressing a long video. My energy saver settings are never for computer sleep and 15 mins for display sleep. Anyone has a suggestion?

Thanks.