I have restored a '71 442. It is a factory a/c car and the air works well. However, after running for about an hour or so, frosty air starts coming out of the registers and then the a/c stops cooling. If you cut it off for just a few minutes, it will work fine for another hour. I know it is freezing up. I had the freon checked (still R12) and it had plenty. What could cause this to happen? POA valve? These old Frigidaire compressors don't cycle on and off do they? Any thoughts? Thanks.