if the ac fan dont kick in then the the head pressure will rise and the gas cannot change state properly and will not be sprayed through the block valve creating the cooling affect also some compressers have a thermal cut out located on the ac pump and can cause the pump to stop. if the clutch cap on the compresser is to big it will not cut back in .what u need to do is get it on an ac machine and read the pressures high side should be about 200psi if it goes above 280 psi fan should cut in low pressure should read about 50psi if it is pulling a vacum on the low side the block valve has had it if the clutch has cut in and pressure is even pump has had it pressure switches can cause your kind of problem over gasing and under gasing also