This could be due to poor air circulation in the attic and home. Without knowing the configuration of the home, attic and orientation of the home it is difficult to recommend a solution.
It would be to your advantage to begin by contacting a home energy auditor to analyze the conditions of your home's construction and insulation as well as check for points of air infiltration. Once they have completed their analysis they will be able to recommend solutions which would best suit your needs.
For more information:
Find an energy auditor near you. There are approximately 2,500 energy auditors and raters across the U.S listed in GreenHomeGuide's Find a Pro section.