a/c blower doesn't work on high speed only

i have been having problems with my control module when running my blower on high...it works for low, med-low, med, but not for high...i bought a used control module and replaced my existing one and this worked for a few months, but am having the same problem again...anyone have any suggestions or experienced similiar problems with their tahoe or yukon...i have a yukon denali but the tahoes (1999) have the same temperature control unit which i have replaced twice...thanks for your time...

Jermaine