Lately, every time we turn on the shower, the toilet bubles. When we try to flush the toilet, it doesn't flush, it just drains slowly. If we don't use the shower - like overnight, the first flush in the morning- the toilet will flush normally for the one time - then it again doesn't flush - it just drains slowly. At time we'll hear air buble in our tub if the toilet flushes.

This indicates a blockage in the drain line. You can go at this one of two ways.
1) You can rent a sewer machine and snake from the lavatory roof vent, **OR**
2) You can pull the toilet and snake from there. If you snake from the roof put out enough snake to reach the base and 16 feet more. Good luck, Tom