I have a 91 240 that on ocasion has over heated,or ran hot on 90+days, overwise worked good, needs new coolant?

Do a Coolant Flush and Refill with a 50/50 mix of distilled water/coolant.

Check the condition of your radiator, if it is loosing the fins it will not work well. It would be a good idea to flush it as well.
Good luck.

Mike