wpc
I would check the vent on the plumbing, also it appears you have a stoppage in the drain. It appears when you have substantial flow the drain starts filling @ with no place for the air to release it raises the water in the bowl, when the water in the drain is being released through the drain slowly and no air being admitted through the vent your water leval would pull down. Check the vent, drain and your septic tank it could also be filling.
