2001 Nissan Frontier A/C only works for 30 minutes when car

My a/c needs repair. When I start the car after sitting all night, the a/c blows cool for maybe twenty minutes. After the engine warms up -- no more a/c. It blows like the heater is on, and in Houston, TX in July it is painful. The clutch is no longer spinning and will not click on. I have taken it to my mechanic but I don't believe he has the proper testing equipment to make a sound diagnosis. I can't afford to replace every part in the system until he finally figures it out.