I have a friend who had a mare with this problem. When she was turned
out and just ridden lightly, she was fine. Anything more serious in the
way of exercise and she would tie up without fail. My friend put her on
the Cornell U. diet for horses with this problem and it helped a great
deal. The mare still was not okay for serious work, so she sold her as a
pleasure horse with full disclosure of the problem.
Ask your vet to find out about the Cornell diet for tying up. It
involves a LOT of oil--something like 2 cups per day.