compressor kicking out???

Hi, this past week, my '89 190E 2.6 has been having AC difficulties. One day, I started the car, the air was on and all was fine. Drove across town and back, no problems. The car sat for about an hour, restarted and drove...no AC. Came home and popped the hood, compressor was not running. Waited about 2 hours, restarted the car, AC working fine. yesterday, same thing...I stuck my gauge to it and it has the correct amount of 134a in it...the electric fans work...the blower works fine.
Last night, I was driving around just listening to it...when I turn on the air, it's cold for a minute, then i hear a "bark" sound and it kicks out. Stop the engine and restart several times and it does this several times...all with the same bark sound...is my compressor trying to lock up?