I put in a brand new fan motor & capaciter that matches perfectly with the one I removed
But it goes off on internal overload after 5 minutes. What could be my problem!

Overheating. Is the coil clean? You never said weather this was a inside motor or out side condenser motor.

Did you use a Amprobe to set the maximum curent draw of the motor. You can have the speed set to high and this will also cause a overload problem.

If this is in fact the condenser fan you are talking about is it possible that you have it wired wrong? Just because a motor may look the same doesn't always mean it is. I would look at the side of the motor there is a diagram showing you what color is what, double check your wiring, if it is wired right it is possible that you got a faulty motor and again I'm assuming you are talking about the condenser fan "on the A/C". Give us a little more info and we'd be glad to help!