You have insufficient air flow to begin with. Check your Soffit vents, did any loose fill block them? How many ridge vents do you have (I think I see one)? Basically, add more passive air flow to the system. If the fan helped alleviate the problem, a long term fix is adding a permanent (passive) fan. If you ignore the vents, you'll probably have this problem again.
Good news, its pretty easy to add more venting if that's the issue. If I'm wrong, the extra venting doesn't hurt.