electrical problem

I have a 2000 f150, and my prodlem started is when I crank it when its cold, it will crank fine. But after i run it a while and go to crank again it wont even turn over, Like the battery is dead, but the battery is fine. Then I wait a few hours try again and its fine. Im afraid to turn it off when i go someplace. Thanks for your help

Starter solenoid is getting heat soaked (or has a bad connection).