AC issue

What's up all. I have an 01 Frontier XE V-6 5 spd. Recently my ac compressor has been cycling off and on too much. It shuts off too much. When the compressor is on it puts out ice cold air. I notice when im idling the engine it's usually ice cold but when i'm driving over about 20-30 mph it loses its coldness. Then when I pull over and check the compressor it's off but then it starts up again a few minutes later and gets ice cold. I recently put in a new thermal amplifier so it's not that. Does anyone know any tests to see if the ac belt is not loose and if the compressor clutch is going bad? Any other ideas?