No A/c Help!!!

89 560SL First it was Intermittent. After starting the car, the AC just would'nt come on. I tried alternateing on and off ac switch and sometimes AC would come on sometimes not, sometimes it would opperate fine othertimes it would cut out after a bit sometimes it would come back on. I even tried (with about the same success ratio) restarting the engine at stop lights. That was last summer. This summer it blows hot air only. It seems to be hotter off idle than at idle. Also the center vents are not as hot as the outside vents???? I have searched the forum whith out any discussion matching my symptoms
Somebody hep me please.....