I've overclocked my gpu @ +315 core clock & +850 memory clock and tested it several times for stability, which proved to be great. I've noticed when i play COD:BO 2, every now and then my msiafterburner core clock graph utility shows minor dips that lasts for about a second (1150 mhz down to maybe 1130 sometimes 970). It doesn't happen often but it does happen. I don't think its throttling because my temps are fine. I don't hit over 64C and my GPU usage graph looks extremely consistent and isn't all over the place.
The only thing i notice in game is it becoming a little sluggish but my frames don't drop at all. They sit at 60 fps while i experience the sluggish (micro stutter?)
I found this (which I was unaware of) at PlayTool.com http://www.playtool.com/pages/troubleshooting/intro.htm...
"...one of the things that changes when you're playing games is how your sound hardware is used. When you're not running a game, the sound hardware only uses a very basic sound interface which is simple and reliable. But once you fire up a game, the sound is often generated using lots of complex calculations running on the sound hardware. It's not uncommon for the advanced part of the sound drivers to be a bit buggy and to cause system crashes. So if you're having crashes during games, it's a good idea to temporarily disable your sound hardware while you're troubleshooting. That way you know for sure that it's not the sound system which is responsible for your problems."
Might be something to look at - I'm currently under the belief that the issue is not related directly to your overclock or graphics card (although a slight voltage bump may be desired) which is why I'm looking at this and other alternatives
I also have a GT 650M in my laptop which I do some gaming on occasionally when I'm on the road. I also see this behavior with some games in certain circumstances.
What you're seeing is the boost clock being dynamically throttled down due to power or thermal constraints. Unlike CPU overclocking, overclocking on Nvidia Kepler GPUs basically just raises the clock speed ceiling for boost, instead of setting a hard clock target at load like CPU overclocking. Your temps are well out of the range of thermal throttling, so it's very likely the GPU boost clock ceiling is not being reached because the GPU is exceeding its rated TDP at such high clocks under certain loads, which forces the boost clock to lower to remain within spec. I'm not sure that there's a way to modify the TDP target of mobile Kepler GPUs, but if so, that's the solution to your issue.