GPU issues.

First of all, I hope this is in the right section. I'm gonna do my best to describe the problem.

In games such as wow wotlk, dragon age, farming simulator 2013, euro truck simulator 2013, need for speed rivals, on high and ultra details, all maxed except anti aliasing, on 1360 x 768 resolution I get between 15, 16 and 30-35 fps most of the time. Sometimes when I'm in a game, I alt tab and when I go back to the game, it suddenly runs on between 60 and 225 fps. So I saw this as an indicator that my card is actually capable of running all those games on ultra details with at least constant 30fps, but if I close the game and open it again, any of these games I mentioned above, it goes back to 15, even single digit fps on those settings. My question is pretty much what's causing this and can I keep the 60-225fps every time I open a game instead of it randomly happening just for a period of time? Hope you understood the problem, my english is a bit rusty...

My pc specs:
AMD FX 4100 Quad-Core 3.6Ghz
AMD Radeon HD 7750 2GB DDR3
6GB RAM, 1.5TB HDD
600w psu

Any help would be much appreciated, 'cause atm feels like it's working worse than a phenom II x4 cpu and ati radeon 4870 gpu, which was my old pc.

Yes, on most games my fps doesn't go above 35 on medium settings, 10-15 on high and less than 10 on ultra. I forgot to mention that all drivers are installed correctly and updated, and I'm using windows 7 x64.