Car overheating

My 1999 buick century was overheating,I took it to
the shop and they said it was the radiator because
the fluid was leaking so we replaced it with a used
one in august and it was ok for a couple weeks
then they said it was the intake manifold gasket
leaking they repaired it at a cost of 600 dollars and
now 2 weeks later its doing the same thing with
fluid leaking fom what looks like the radiator so the
mechanic said it's a hairline crack in the radiator
and put in a brand new one but then calls me and
said it's a blown head gasket but there weren't any
of the indicators of one I saw no oil contamination
or white smoke etc. What could be going on?