Could be a broken neutral if the apartment is on a dual voltage system. There are 3 wires feeding your panel - 1 neutral and 2 hot. Each hot should be 120v to the neutral and 240v to eachother. If the neutral is broken then the voltage will go crazy under unbalanced load. The more unbalanced the load on the 2 hot legs the worse the voltage will be. The voltage will always equal 240 (say 90v and 150v for example). Open your panel and take voltage on the 2 hot legs while turning things off and on and trying to make the load unbalanced between them. If your voltage goes weird then you have a broken neutral somewhere source side of there. Don't try to fix it unless you've shut off all load because technically that neutral is the return path.