a/c blows cold then no air

[rusvd]

This is a 94 f250. The a/c blows cold but on trips after about 45 minutes or so it is not cold anymore. The fan is blowing but the air is no longer coming out of the vents anywhere? If i turn the system off 10 to 15 minutes then i can turn the a/c back on only to have to go through cycle again farther down the road. I dont notice this around town, only on the longer trips when i'm towing and the weather temp is up 90 degress more or less.
Any ideas what i can check out.

Thanks.............Jason

[Bob Ayers]

It sounds like you are low on charge, and your evaporator is freezing up. Have the charge level checked.

[rusvd]

Thanks Bob, i will have the a/c charged checked this week.

Jason