Might want to look at replacing the compressor. Shouldn't be too pricey to have your AC repairman take care of that for you. While we're on the subject, when you DO replace your central air unit, look for one that uses R-410A (Puron) instead of Freon. It will cost a little more, but in the long run they are substantially cheaper to operate and far more environmentally safe. Also, after 2010 new units will not longer be allowed to be manufactured that use Freon. Just an FYI.