1970 C10 A/C not blowing cold after 60 MPH

[bille1ear]

Thanks for all the great ideas.
I have a manual valve on the heater hose that is off. It didn't take long for me to realize the heater valve was leaking in the Texas heat. So I doubt that's my problem.
The cool air comes back quickly when I drop the speed so I don't think freezing is my problem.
I was thinking overcharge when I posted. My gauge set is from the Freon 12 days and the hoses are shot, so I need to get one of those quick charge kits and see if it's in the red.
Will post back when I have an answer.

Thanks again...