Bitt,

We had problems last year with a split evaporator/condenser unit over the course of several warm months. They kept recharging the refrigerant loop to no avail. Finally a frustrated tech drained/purged the loop (my approximation of his explanation) then recharged it and all was well, relatively-speaking (we replaced that 1.5 ton unit in January with a new 5 ton).

So the tech's take home message was a "bubble" -- a locked up loop that the system was laboring against. It took 4 service visits. The first 3 were "I charged it. Seems fine!"