Replace your cold control switch. It is the box under the passenger side dash, it has an aluminum wire going into the evaporator. It sends a signal to the compressor to shut off when the evaporator gets too cold. When they start going out they can work intermittently, causing your problem.

 

check for 12 volts at the compressor. turn key to on, (engine not running) turn a/c on, fan on, and check down at the compressor for power. this could be your air gap on your clutch becoming to great for the coil to pull the clutch in. if you do have power going to the compressor, this would indicate your clutch is going bad.

edit: i think you will have to have the engine running to check power at the clutch.....if i am not mistaken. toyotas are a little different (for the better) than what i normally work on.