I am on my 4th radiator and it still leaks. Why?

My local mechanic has been installing them - the first 2 came from the same place. One leaked from a bad seam on the bottom of the radiator and the second couldn't hold any pressure and gushed antifreeze from the lid. Tried 2 different caps and finally took it back to mechanic. They installed the 3rd from a different vendor and that is slowly leaking from the "top"/cap and possibly a hose connection. Mechanic even though he tested for head gasket issues - said it MUST be the head gasket now since the radiator can't handle the pressure after being run for 10 minutes.

bad cap ain't gonna kill 4 radiators!

It sure can. System needs to be under high pressure to cool properly