If this is a new behavior try to think about what has changed. It could have something to do with a dietary change, change in pasture mates, change of bit, saddle, amount of work, etc,. If your major problem is at the trot try to do exercises with trotting poles that make the horse think about tempo and where she is placing her feet. Also, since your leasing the owner may have tips about the horse in specific. Or if you take lessons its always different to see what the horse/rider is doing in person than to just read about it on-line. Goodluck!