I have just purchased a house that was built in the late 40's. An addition was added onto the house ( livingroom, bedroom and full bath) My problem lies in the toilet in the main bathroom... When the bathtub, or washing washine are being used the toilet water level drains to almost nothing, and large bubbles come up, and it will not flush... However, when there is no other water running or draining in the house, the toilet flushes fine!

Bubbling or gurgling? Bubbles most frequently mean a clog but that siphoning of the toilet could also be a clogged vent stack. I'd rod down the stack with enough snake to reach roof top to basement floor plus 20'