water spurting out of toilet bowl when flushed

I had a Kohler Cimarron toilet installed by a plumber two years ago. Since then I have had three ongoing problems. The first is a bad smell (like raw sewage) coming from the toilet. The second is any time I flush the toilet the pressure causes some water to spurt out of the toilet bowl onto the floor (which I constantly need to clean). The third problem occurs about once a month and I would describe as a 'hard' flush where it sounds like air is being forced through the pipes.

Any ideas?