If you say your cold air was 81, and your warm air was 71 at 3' downwind of the unit, that doesn't sound right to me. That's only a 10 degree cross. Anytime I take a cross in a home, I typically go 3 to 4 foot off the plenum, and look for a cross of 17-21 degrees. You're only getting ten. Is you filter and coil clean? Is there a chance your coil froze up? You said it was quite hot out, so it must have been running a lot. Good chance of it freezing. Also, what speed was your fan on when you took the cross? That makes a difference too.
I cleaned the filter two months ago but will clean it again tomorrow. I've never cleaned the coils so I can only imagine they need cleaning. Not sure how to do that though. The AC had been running non stop since about 11 and we returned about 6. 7 hours maybe more.
I set my thermostat to the temp I like and during the day it works toward but doesn't reach that temp until after sunset and then cycles off and on during the night. Should I be setting the thermostat much higher so that it has a chance reach the set temp and cycle off and back on?
When I was doing the vent temp check the fan was on high. Since changing to the digital Hunter thermostate I don't have a low fan setting anymore.
When you get a chance, climb on top and pop the cover off. Good chance your coil is dirty. If it is, good chance you are icing up. If it happens again, shut the AC off for a couple hours, but leave the fan running. If it is iced, this will thaw it. Then, when you turn it back on, you should get some cool air till it ices again. If it is icing, and the filter and coil is clean, then next likely reason would be an undercharge. AKA, loss of freon. But, that'll be our last resort.
Ditto on looking carefully at the ducts. When I did, I found duct tape dried, fallen loose, and both allowing leaks and obstructing the air flow.
The fix cost nothing, took 5 minutes (2 spent shaking head at the connections' flimsiness) and made a big difference.