Bravada A/C with a mind of its own


The A/C in my 1999 Olds Bravada is acting strangely. At cooler ambient temps (60’s or so) it seems to come on fine. Other times when I hit the A/C switch the light in it comes on and off 3 times and the A/C does not come on. However, if I drive around 1.5 miles it comes on and seems to work fine, plenty of cold air etc. I had it in the shop for 2 days with no luck. They told me there was plenty of refrigerant. Can someone help me?