Have a 2003 F150 v6 4.2 L change the lower manifold intake gasket and put it back together and it drove good at the beginning and then horribly had no power what would the problem be

 

The first thing you should consider is to, RECHECK ALL YOUR WORK. Even experienced techs go over their work to make sure nothing was overlooked.

Pull a spark plug and see what it looks like?

That can tell us a lot about if sufficient or excess fuel is getting into the combustion chamber.

 

Use a scanner and monitor the fuel trims at idle, 2000rpm, record for ~30sec in graph format and post, we can then see what the engine is doing..

 

We need to keep in mind a lot of folks asking questions are not mechanics nor do they have expensive tools and equip.

Some just like to appear as if they know a lot about the vehicle and there is nothing wrong with that.

 

I do keep that in mind, but if you want to CYA and confirm the fix without guessing you either pay a mechanic or invest into some tools..

And, I an not into the guessing game, nor do I have a crystal ball.