Toilet flush problems

When we flush the toilet, everything goes down just fine the way it is supposed to…the tank and the bowl then fill up with water and the water shuts off the way it is supposed to…then with about 30-45 minutes, the water in the bowl drops about 1-2 inches and then stops dropping and maintain the new lower level…what might the problem be?One person suggested that the vent is clogged by leaves or a birds nest or something causing a vacuum with the water in the bowl. Someone else suggested that there is a crack in the bowl and when the water drops below the crack, it stops…we do not see or feel any cracks…is there some way to be sure of that before we go to the expense of replacing the toilet?Thanks for any help you can share!