But my FPGA still runs fine at 600MHz.
I guess it's stretching the limits. Also it could vary chip to chip, since -3 speed grade has a Fmax of 601.32MHz
Did you just modify the clock on fpgaminers code? I upped the clock to 450mhz and it was running stable at 70C. The chip is rated to 85C, what temperature were you at at 600mhz