Re: whats the problem
Will it cool to 75F when it's 90F outside? Are you keeping it at the same temperature all day or using setbacks Design temp in most of New Jersey is only 89-91F. At 95F it should only hold about 77F indoors.
Are most of your blinds closed? Especially on the south and west?
Sounds like it's sized almost perfectly. Better to be slightly undersides than oversized. Long run times make a system keep humidity lower and run more efficiently. It takes 8-10 minutes to reach maximum effceincy. The systems in many homes rarely run more than 15 minutes at a time.
I agree that adding insulation may help. Sealing up air leaks will help the most. Door seals, storms doors, window seals, sill plate, and most important, any penetration into the attic. A larger unit will be noisier, use more energy and not remove humidity as well.
FYI - design temperature in Florida is no higher than where I am in SE Iowa... only the humidity is slightly higher... and it stays warmer year round in Florida...ie you have no winter.
1925 Two-Story Stucco Beaux Arts Neoclassical