Had 4 techs come out and no one can fix the problem. My thermostat honeywell 8000 is set to 76 and the temp stays at 80 when its over 90 outside. At night and during cool days it cools perfectly fine. The coil is in great condition from what all techs have told me, filter is good, one tech did sub cooling on it and he said it was good( at 15 dgr I believe) The TXV is working fine and the ductwork is all in decent shape.
Temparature at the return is at 78 and at the registers its at 59.

I am running out of ideas and of money! What else could be the problem.

The house is 1500 sq ft and the outside unit is a 3.5 ton brand new. Inside air handler is 3.0 ton ruud about 8 yrs old.

Is it possible the thermostat is bad/ But it cools great when temperatures arnt high!

Should I replace air handler instead of having another tachnician take m money and tell me the same thing??

Please HELP