This was a timely article: my gelding is developing the same attitude toward flatwork. Thank you for the suggestions!
As usual, Ms. Hale provides great advice here. One thing to consider is: How does the rider feel about flatwork? If she becomes bored or impatient while riding on the flat, the horse may be reflecting this attitude by "misbehaving." I would suggest that the rider ask her trainer about doing some cavalletti work to provide some spice to her usual flatwork regimen. While cavalletti are not jumps, they provide a similar physical and mental challenge: The horse needs to be forward in order to negotiate the cavalletti. Meanwhile, the rider must work on her balance, equitation and communication with the horse to help relax and encourage her mare to successfully negotiate these obstacles. If boredom is an issue for either of them during flatwork, cavalletti exercises will reinforce the need to maintain even stride and cadence while approaching each obstacle without jumping any of the poles.