In the mornjng when the outside temp is 75 degrees the suction pressure is about 70 psi and the temp coming out of the vents is 54 degrees. In the afternoon when the outside temp is 96 degrees the suction pressure goes up to 90 and the temp coming out of the vents is 66 and the thermostat will not get below 76 degrees. What might be the problem.

Check to see what refrigerant the system uses. R22 or R410a. It will be on the nameplate on the outside unit.