2000 Dodge Dakota Stops blowing cold after about 30 min of

A/C blows cold for about 30 min of highway drive time, then starts to blow at about 73 degrees. Happens EVERY time I drive it. The compressor cluch does engage but never cycles on or off, is it supposed to? I have checked the compressor clutch relay and all seems to be fine with that. I have found that if I turn the A/C on and off periodically while driving (5 min on - 5 min off), it will continue to blow cold when I turn it back on. Have any ideas???