interior fan problem

i have a 2003 yukon. Ac and heater work great, just the problem i am having is that the fan doesn't really start to work until i have it set to position 3. Also if i have it set to 2 and i an driving about 45 miles n hr you can start to feel it, but when i come to a stop then its like it is off. I don't know a whole lot about cars, i did check the fuses and they are good. Any help would be great.

Thanks in advance, lewis b.

i've known several people with this prob. all the 90 models had fan probs, didnt know it still was an issue. u may think i'm crazy but kick/ hit the fan box and it will come on. (the big black box on pass side under dash)