I've searched and read through numerous other posts however I haven't been able to find anything exactly like my problem.
We recently moved and the furnace and central a/c are both very old. Furnace is probably 30+yrs and a/c is probably 15+yrs, however they both 'seemed' to work ok. The furnace got us through this winter, with the exception of the blower motor failing in early March. A local place came out and replaced it with a used motor and things continued working just fine.
This April I tried using the a/c for the first time and it kicked on fine, but I noticed about 15min later the blower wasn't on anymore, but the a/c was still running outside. I could see frost on the line outside and I quickly learned that the coil freezes up when the blower isn't running. I tested it out several more times and each time it would start running but then shut off within an hour or so. This led me to believe there was some problem with the system not realizing that the temperature hadn't yet reached our set temperature (so maybe it was still 76 degrees in the house and I had it set to 72) and so it was telling the blower to shut off.
I then tried running it with the fan set to 'on'. The blower always turns on however it shuts off at totally random times thereafter. Sometimes it only runs for 5 or 10min, and other times like just a few days ago, the blower ran continuously for about 3 days straight. I'm sure its just irony, but it seems like it shuts off more frequently on hot days and then says 'Oh hey I can run for you' on the not-too-hot days.
So, with that said, what would your recommendations be on my problem? Despite the age of the system, when the blower decides to run, the a/c can manage to keep the first floor of our house comfortable even when temps climb into the 90's, so if we can salvage another year or two out of the extremely dated system then that would be great. The easy but potentially expensive option is to call a technician - I am willing to do some tests or maybe even clean the blower out if it could potentially solve the problem.