toilet problems

We are having trouble with two of the toilets in the house. Both of these toilets come close to overflowing frequently and are slow to drain. They also clog very easily. The water level gets really high when they are flushed but they have only overflowed once. We plunged the toilets and used a toilet auger but the problem still persists.

Since both of these toilets have the same problem, we were concerned that we might have a problem with our septic system. The other toilet and other drains are all working fine in the house though. Does anyone have any suggestions on what could be causing this?

Make & model, plus year of installation? This would be abnormal for some toilets, normal for others.