a/c not working

I have a 95 chevy blazer the a/c is full of R-134A the compresor is cycing. it feels like the heater is on all the time if you switch the dial to vent hot air still comes out of the vents. when you switch the max a/c the door on the heater core opens and the air blows a little cooler but still about 100 on a 90 day not so good any ideas