I have found that if you are low on refrigerant, the system tends to freeze up. It will work for a bit and then you have to turn it off for a few and wait for it to thaw out. I know that everyone can't do their own work, but that Jiffy Lube was really over charging. A can of 134 around here is $7 figure on 2 cans and good stop leak is $20 (best to replace the leaking part) So they really jacked you on the 143a.
I just replaced the compressor on my car, bought a venture vacuum pump at Harbor Freight for $14, 2 cans of 134a and a can of 134a with PAG oil. I had the hose with pressure guage. Cost me around $35 and it was easy to do. Blows 40 degree air. Of course my car uses 134a so no conversion was necessary .
I had a '91 T-bird that I had to do a conversion on, I flushed it out with compressed air as best as I could then added the new conversion fitting and vacuumed it down, added a can of oil/134a and charged it. It worked ok for the most part, but it was never as good as it was on R12 and not as good as a real R134a system.