Carbureutor Problems

I have a 2003 YZ 250 that has always ran great. Last weekend at Clear Creek the carburuetor started leaking fuel out the overflow tube that drains from the bottom of the carb. Whenever the gas is on, gas just pours out the overflow tube. I was able to shut the gas off and start the bike, then ride it but it did not run well and constantly leaked gas. I took the carb apart and cleaned it (did not see any dirt or obvious signs of wear). The only thing that I can think is that the float is sticking but when I took it apart the float seemed to be working fine. After taking the carb apart and cleaning it and putting it back together it ran good for a few hours but then started leaking gas again. I am going to tear it apart again and see if I can find anything wrong but I would appreciate any ideas. Is there anything I should look at other than the float? what would cause the float to stick intermittently?

thanks,
Jim

thanks for all the information. I have the carb apart right now and have inspected all the parts and cleaned everything again and all looks good. What is the best way to measure the float height? My book says to hold the carb upside down and measure the distance between where the float meets the carb mating surface and the top of the float. It seems like there would have to be fuel in the carb for this to work? I would appreciate any help.
-Jim