Central Air Repair

Our Central Air unit (we have the kind that snakes through the house from the basement like a spider — forgive my ignorance, can’t remember the name) has not been working this season. The people who installed it with whom up until now we have been quite happy, are just not getting it fixed. They come over 2-3 times a week to “find the leak” or “bleed the system” or do whatever it is that they think is wrong with it and of course only several hours after they leave does it stop working. Then it takes several unanswered phone calls to get them back. They keep saying that we have to bleed the unit until all the air is out but we can’t seem to get it to work. We are hesitant to fire them and start from scratch with a new company, since we suspect that that will me quite expensive. Any ideas? If we should cut our losses, does anyone know of a reliable, responsive central air repair service?