Call an HVAC company. And there's a difference (I learned recently) between closing vents and using dampers (I think that's what they're called) on the main duct work out of the HVAC unit. Unfortunately, these are often unmarked so you should ask the HVAC person to show you. I've also heard that an attic or roof fan (different than a ceiling fan) helps.
We have this same problem and it is sooooo frustrating. I've heard that energy audits, which find places in your house lacking insulation and other problems, can help with this too.