How do i fix my movie downloading problems? I bought some tv show episodes and a couple movies on iTunes, but when I try to download them on my iPhone 3GS it downloaded one movie then says "You can download past purchases on this device with just one apple ID every 90 days." What do I do?
You need to wait 90 days and then sign in to your Apple ID on that device. The words from Apple on this matter at this link. 

This prevents people from sharing their ID with many devices or many ID with one device to avoid purchasing content on the stores, I believe.

You can find out how long you have to wait by opening iTunes on a computer and signing in to your Account Information.

From there, choose Manage Devices: