here is the story. i am diving and i turn my ac on for a bit(i had been using the ac for the past couple weeks. i start noticing my car getting sluggish i get home and hear hissing. sure enough my over flow for my radiator is spitting out steam and boiling coolant. i cool it down and check my cap...its broken into pieces(i had this type of problem in my old honda, but i replaced the cap and it was fine)
so i got a new cap but upon removing the old one a small plastic piece went into the radiator and is still in the cooling system.
my radiator holds pressure but jumps to overheating in less than a mile.
here is what i am guessing.
A- the plastic piece is ubstructing the flow or stuck by the thermostat.
B- bad thermostat
C-blown gasket
d-bad radiator

what is the best way to go about narrowing it down cost wise???

and how can i get the piece out, will it come out if i drain it or should i flush the system??