Calling all BMW Experts

Heres the situation now. I have a 93 325I. When going to work in the a.m. I have to add about a cup full of coolant to reach the correct mark on the expansion tank. When I leave work in the afternoon I check it and its always at the right level. When I get home I let the car cool down and check it again and the coolant is damn near at the top of the expansion tank and for me to bring the coolant back down to the right level is to bleed the system and thats when I leave in the next morning I have to add more coolant..The needle is at the correct mark and sometimes it will go very slightly over the mark and then drop right back to the middle. The needle doesnt ever reach the 3/4 mark, there are no apparent leaks..Could this be a bad rad, a bad stat or something else Im not aware of..Help..Its about 75-80 degrees at 7:00 am and when I leave to go home its about 98-100 degrees and I drive pretty far to work..HELP