I am sure you may have checked but is your air filter clean?
Is your air filter a good pleated filter? If you have not been using a good pleated filter the evaporator coil may need cleaning.
Is it only lately that you are noticing the difference with the house is getting warmer?
If you only bought the house last year the unit may be under sized as last year was a cooler year and this year is much warmer so you will notice problem times more so this year then last. A pro will be able to advise. When you have the pro come out I would ask if the unit is sized properly. What makes you think the unit is freezing up? It should not unless there is a restriction in air flow or the system is low on refrigerant.
Your supply and return differential temps you stated sound ok. 14 deg across the coil is not bad. Not sure what you particular is rated for.
Taping up duct leaks is always helpful if you can access the duct. Do as much as you can reach or get the pro to do it if he is doing some work on the unit.
Hope this helps