A/C will run for about 20 minutes, then will stop, or barely blows out any air?

I have had my blower motor and resistors replaced. I've had it in the shop approximately seven times and each time they just keep changing the blower motor and resistors. Obviously, this isn't the issue. What do I need to have looked at? Please help. I'm a single mom and can't keep putting my car in the shop and get the same runaround over and over.