I've got my 420m overclocked to 740MHz, stock is 500Mhz. I believe I'm going from a 420m to 540m in core clocks. I have yet to run into a problem. What I cannot do is overclock the vid memory. Either the vid memory isn't cooled or it's bad for the chip/motherboard. I had it overclocked from stock 800Mhz to 900Mhz which is the speed of a 540m and it degraded the video memory. I cannot overclock the video memory anymore. I'll get a ton of errors in OCCT.
With my cooling pad I'll hit 70C in BF3 with the gpu overclocked 48% over stock clocks.
Try to use OCCT and figure out if its vid memory or core clock. Run the mem test if you can.
You should be able to at least get 740Mhz. Try MSI afterburner and EVGA precision. Whats odd is Afterburner lets me clock to 650Mhz max, EVGA precision lets me clock to 800Mhz on the core.
I personally like OCCT for testing my laptop. Desktop, maybe not so much since I overvolt.
Hope that helps