Air Conditioning Issues

I have a 91 Accord. I recently (a month or two ago), changed the refrigerant from R12 to 134a with one of those $60 kits from AutoZone. I am 95% certain that all of the R12 was out of the system. When I first did the conversion the air conditioning worked great. After a week or two, the air would sometimes blow hot, sometimes blow cold. It seems that when I do not use the AC for a while, it tends to blow colder. The system has/had (haven't checked recently) the recommended 25-45 PSI of refrigerant, but it would still occasionally be hot air, even when after I double checked the PSI. A few times I was driving at a maintained speed, and it just kicked in with colder air. I know its not the blower because it maintains the same stream of air the whole time, air temperature is the problem. Any suggestions would be appreciated.