Hey guys, have a tractor that ac works perfectly when running around the yard and light duty. Hook my baler on and sometimes will get 2 hours sometimes only 20 minutes until compressor kicks out. Usually can just shut of fan and turn back on and it will kick in again. Have had it into a shop twice and changed expansion valve and low pressure and high pressure switch. Pinched off heater hoses and no change, system is charged. What else can I look for?