strogg wrote: ↑Mon Jun 29, 2020 10:27 pm Thanks everyone for your input so far. I've learned a lot despite my brain being over capacity as it was.
I took the temperatures of the supply and return grills with my trusty IR thermometer when the AC was operating. It was an 17-18 degree delta at its coolest (54 and 72 degrees). The filters are still clean, the outdoor units were also clean, and the coils were clean the last time they were checked a few months back. I can check in the morning when it's not so hot in the attic.
I also noticed something stupid that I can't believe I didn't piece together for the past 3.5 years I've lived here. As I was adjusting the downstairs t-stat again, I felt a breeze on my hand... The thermostat is about 6 feet diagonally from a supply duct. A little bit of cold air was blowing right at it. I'm going to move the thermostat to the other side of the wall in the morning. That side has pretty stagnant air and is also more representative of the actual temperature of the zone. The upstairs one is also 6 feet diagonally from the register, but I placed a barrier in between the two a year and a half ago, so that's no longer a real problem.
This may be a big part of your issue. At one facility I worked I could not get consistent heating/ cooling from one unit. I had three different HVAC techs out and they were unable to solve the problem. I decided the t-stat was not accurately reading temps and went to replace it. Whoever had installed it had cut a huge hole in the wall, so it was sucking air into the back of the t-stat. Hot in the summer and cold in the winter. I filled the hole with insulation, and voila, problem solved. Good luck.