air conditioner on 2001 Ford F-350 Super Duty

when I start my truck up the a/c runs fine cooling very good. after running a while it continues to run but does not cool. However, if I slow down it begins to cool again and if I acclerate it stops cooling again. What can cause this.

Assuming that the A/C system is not low on refrigerant, one likely concern that we have seen on this model is the A/C clutch gap being too wide. When the system commands the clutch to energize, it can not due to the excessive gap.