A nice chap from the AC repair company I called came out and diagnosed the problem with our AC. It was the capacitor/start kit for the high side of the compressor. Unlike many typical central AC units ours is a 2 speed so when the demand for cool air is low the low side of the compressor runs and when the demand increases it shuts off that side and kicks on the high side. That's why it seemed to cool when it wasn't so hot outside and at night. He measured the microferad reading on the capacitor and it was way off from the specification. He thought it was an original part which makes me feel a bit better as the unit was installed just about 9 years ago and we have replaced the low side capacitor in 2013. The cost was $225, $175 for the part and $50 for the service call. This was a smaller AC company than the other ones we usually call on, the others charge $85 for the trip plus labor on site. He checked freon and other things and said it should be fine now.
