my3rockyme

I have a 1992 Lexus LS400. It has over 250k miles. I'm having trouble with it overheating. I have to add coolant at least once a week to help avoid overheating. When it gets hot there is steam coming from the top of the reservoir tank. Other than the steam I don't see the coolant leak from the car, so I'm not sure if it is leaking while I'm driving. It will overheat when idling at a restauarnt drive-thru for 5 minutes or if I'm at a stop light for a couple of minutes while the a/c is on it will start going up. I have been told to replace the radiator and now after reading it seems like I should replace the reservoir as well. Any suggestions???? And, as far as I know my head gaskets aren't blown.