So the previous owner replaced the radiator . It's aluminum but with plastic ends. I can't keep the cap from leaking.Tried 2 different caps. Should I get an all aluminum radiator?

 

Got any pictures of the top of the radiator where the cap rests and the cap itself? Is it perhaps just not seated properly?

 

The leak may because the top it is attached to is not flat. Maybe damaged??? Lay a flat plate on the top of the opening and see if there are gaps. If so maybe you can bend or file flat? Make sure the cap is snug, check the lower part of the interface to assure nothing is bent out of wack if the cap is not snug
if problem persists please post some close up photos of the opening
also maybe use a marker like chalk on the rim of opening, then put the cap on, then remove cap and examine both the rubber seal in the cap and the top for signs of uneven seal

another option is maybe use two gaskets on the cap, although that may make it difficult to screw on so you can maybe get some thing gasket material, cut yo fit and add it under the stock cap gasket

but first assure the opening seal surface is smooth and flat, no gaps when you lay something flat on it. Perhaps a small mirror or flat sheet of metal.

 

Check the coolant overflow hose for blockage.