The problem you have may well be a blockage outside the house rather than with the toilet itself.
Every few months or so I notice the water in my toilet bowl rising further up the pan than usual - then it slowly sinks to its usual level.
Outside is an inspection pit where all the drains meet and run off to the main sewer in the street. This is where I always find the blockage - the pipe that runs to the main drain.
I fix it by using a stick to pull the blockage back into the pit - I reason that trying to push the obstruction 15 yards up the pipe would be too risky and likely to make things worse.
Of course - you may just have a blocked S bend on your toilet bowl - something that's never happened to me.
Good luck...
Peccavi
April 2013