This can be one of three basic things, maybe more.
Low freon- as engine speed increases the suction (low) side pressure drops, the switch opens and the pump shuts down.
High freon- as engine speed increases head pressure (high side) goes up and THAT switch shuts pump down. A non-functional aux fan will also cause this, but at 50 mph that may be moot.
Contamanants- any moisture will raise head pressure. Replace drier (accumulator), evacuate and recharge with fresh ester oil and freon.
Pressures should be: Low- around 45psi, High- 150-200psi. Any extreme variation in these pressures may signal something else such as a restriction...Cal