Ok so the other day I was in my room bored and figured what the heck I'll download afterburner and see if I can overclock my gt640m. So I got afterburner installed and found it will let me up my core clock and my mem clock. At this point I ran the 3dmark sky diver test to get a base score, I averaged 20fps without overclocking. Next I overclocked my gt640m +135 on the core clock and +1000 on the mem clock and ran the test again, this time I averaged 24fps. I ran the test several more times and all was good no crashing or anything. During those test it got up to around 76c but never throttled. After a few days though I'm not passing 70c but yet I'm only getting 405MHz on my core clock and 800MHz on my mem clock. I ran 3dmark again and only averaged 13fps. I have no idea what is causing this, I thought it my be heat but as I said its not even getting as hot as it did the other day. I did undo the overclock at one point but I had the same problem. The base core clock is 625MHz so I should be getting up to 760MHz now. Nvidia doesn't list the mem clock on the spec page and I didn't watch it the other day to see where it was at before overclocking so I don't know what I should be getting up to on it. I'd appreciate any help I can get to fix this.
My highest 3dmark score was achieved on a lower than max OC. Also this is a shot in the dark but maybe its due to power consumption laptops don't have a ton of power headroom you also could have damaged it.
It might just be that while you are wanting to get to that higher threshold of performance, the heat is most likely holding you back. Basically thermal throttling is holding back your GPU. This is my assumption, could be the simple fact that the bios written to run at those specs.
It seems it was a issue with either the driver or afterburner. After uninstalling both and re-installing them it works fine now. Thanks for the help.