I have a 1998 Windstar with a rebuilt trans with around 30k miles on it. It shifts fine until it gets hot then it doesnt want to shift into 2nd gear, it will go from 1st to 3rd and then the od light starts blinking and it wont go into od after that either

. I am going to change fluid and filter but would like to know what would cause this problem so I can fix it while the fluid is out if it is an internal problem. The warranty is up on trans so am trying to fix myself. Thanks in advance for your help.