1995 Other Alfa Romeo Models Alfa 168 Super 3.0 V6

Hi there,
1) I am loosing some coolant in the radiator sapre tank every week about 1liter. And I have changed the thermostat to work at 75 Deg C. And still when driving in city and traffic jams the temp. Still reaches 90 Deg C.
2) Secondly, recently my fuel injector was leaking fuel out to the engine and I got it replaced with used one, since than the engine is not so smooth and the idling speed hold on to about 1000RPM for a couple of seconds before it reaches to about 700 RPm which is normal. Could be the replaced injector is dirty or malfunction?

Thats all.