Thank you Coding Badly. That was valuable information. I have been running the board on 3.3V for some time by supplying the voltage to the 5V pin. So far it is working running on 16MHz, but i will do some tests running with the internal 8MHz resonator instead.
I know that there is a 3.3V pin on the board, but powering peripherals with 3.3V doesn't help when i need to do for example SPI with them.
/Magnus