If you are not loosing coolant anywhere, & you replaced the thermostat! Make sure you installed the t/s in the right direction: UP! You can check this by feeling the difference in temp. in the upper & lower radiator hoses. If the upper hose is boiling hot & the lower is not hot, then the thermostat is not opening, they should feel the same temp. Also make sure your coolant fan is working properly, meaning the clutch fan, it should be turning as fast as the belt & pulley! note: if you have an electric fan, ck that too! also when bleeding the system it helps sometimes to squeeze the upper & lower hoses to get any remaining air pockets out!!!