If it's your RAM, then you just need to decrease your RAM multiplier. Always change the RAM multiplier to the slowest speed when you start overclocking. After the CPU is stable, you can up the RAM speed. If it is your RAM, then it would have to be 667 stuff if it can't handle 781MHz. If that's the case, a multiplier of 2 will give you 625MHz, which shouldn't be an issue. You can also up the RAM voltage a little (stay below 2.0V for now, maybe lower unless your RAM reads a higher voltage on the stick).
Keep an eye on temps, you don't want to go over 60C or so for extended periods and pretty much shut it down immediately if it touches 70C or maybe a touch sooner than that. Remember to manually set your voltages. Auto will tend to use high voltages when you overclock. And you shouldn't need much if any voltage increases to get 3.0GHz on a decent mobo.
Did reapplying the thermal paste help with the temps? Also, if you want to check if it's the RAM, download Memtest86+ and run it from a flashstick on bootup.
Here's some instructions on how to apply the thermal paste: http://www.arcticsilver.com/pdf/appmeth/int/hl/intel_app_method_horizontal_line_v1.1.pdf
They also have general Intel and AMD guides at that website: http://www.arcticsilver.com/intel_application_method.html#