i'd check compression on that motor, but i had 1990 xj that was the same way, it had around 150k, and i did everything you did, and it didn't change. it never got really nice heat in the winter.
engine might be tired, thats the only thing i can think of that you haven't covered, as long as you used a proper antifreeze mix, too strong a mix will give you this result also.
edit: just thought of this, if you're heat is satisfactory i wouldn't worry, i would aslo make sure the temp gauge is accurate if you have good heat. but i'm assuming your heat isn't working so good, and the temp gauge is probably right.