I have had a repair man out 4 times and he says he can't find anything wrong with the unit. I cannot even feel air coming out of the diffusers unless I climb on a chair and put my hand right next to them. The A/C runs non-stop when it's hot. It will run for 5-6 hours and shut off while the temp in the house is 79 and the thermostat is set at 74. What could be causing this? We have had coolant added. The a/c man says the coils are clean. I can feel air being drawn into the unit at the intake.