No cool air

I just purchased a 2000 mustang gt 4.6. It has 76,000 miles. When I turn on the a/c there is no cool air.
the compressor will start and run for about 2 sec and then stops for about 30 sec and restarts.
i have the guages on the high pressure side will go to 125 then cuts off and when it drops to 100 the clutch will start again for about 2 sec and the pressure is back to 125 and stops again.
i have the 134 can connected and there dont seem to be any that goes into the system the gage on the can stays in the green and it says its low. When the clutch starts the gage goes to the top of the green and then when it stops it goes back to the center of the green.
any ideas?