Well first off your other games has been below 60 FPS, I have owned the GTX 660 myself and I can tell that you won't get 60 FPS at all times in those games with the highest setttings still great performance though.
To your problem. The GTX 660 should take on Skyrim with ease. I made a video of the performance in Skyrim with GTX 660. http://www.youtube.com/watch?v=jZPAxPEmRi4
Have you made sure that you haven't limited your FPS in any way?
Mods are killers, they really tear on the performance. If you got too demanding mods, your FPS will decrease. If you look at my video, then I posted the average FPS with mods and without. It's about 50% difference.
Have you tried reinstalling your drivers? Sometimes the drivers can really mess up your performance.
I hope you get this problem fixed soon, good luck!