hi,my name is XXXXX XXXXX I will help you..the thermostat,most likely is not the issue here .
these years had issues with cracked heads and or gaskets and a block test should be done ..if it overheats in say 7-10 minutes then moost likely the heads are cracked .
the block test consists of a tube filled with a blue lt yellow if the presense of hydro carbons is present in the cooling system.
a lot of times it may not show white smoke out of the tail pipe .
the tube is placed over the fill resivior a(warmed motor ) and the bulb on it is squeezed to draw in fumes ,if the color turns then the heads need to be takken of and sent to a machine shop for further checking ..
the system (cooling) my also be low and causing the very quick over heating as well but before a block test should be filled up to par ...jazzmaster ...
Say it is a bad head or bad gasket. How much would you estimate the repair cost to be? Thank you for your time.