Before I bought my 72, the previous owner replaced that A/C compressor. After this was done, it would blow ice cold for a while and than would start blowing hot air after about 10 min. If I shut off the A/C for about 30 sec, it will blow ice cold again. After looking taking some time and trying to do some trouble shooting in my garage, I found that the lines up by the fire wall were iceing up. I am thinking I have some type of blockage in the system but can not seem to narrow it down. Any one have an idea they can through my way? I hope to find the resolution soon. Living in Arizona with out a/c during the summer is not a very good idea if you know what I mean.

Find where the frost is and you've found the restriction. Also some STV/VIR systems tended to freeze if they were low on charge.

try a new drier also-some times when the dessicant becomes saturated it will spit some out then freeze a little further down line-if this is the cause it will usually occour around the same place or at the expansion valve oriface. especially if the system was open for a few weeks

also check your evaporator drain tube-if the moisture is building up around it, it will turn the water to ice and block the evaporator from doing its thing-removing heat