Before you take to step to replumb, check your vent lines first. A birds nest there could cause such a problem, in which case replumbing would not solve anything. The second step would be to use a plumber's snake to open the line and free it of blockage. Sometimes a caustic or acidic mixture can cut through the blockage, but if you have a septic, I'd rather not see that stuff get into it if that can be avoided. Those are the two most common drain line problems that I see and they can be cheaply fixed.