The dreaded tying up sydrome. Can anyone help?

["guest@endurance.net" <guest@endurance.net>]

BPrunty bprunty@adams .net
Looked in the archives and didn't find a lot in reference to this. If it is there please bear with a new ridecamp user. After finally scrimping together enough money to buy my next endurance horse I find myself with all the right breeding, all the right P & Rs, and all the right bone structure etc etc and tying up syndrome. To say this is a disappointment for me is a gross understatement! Rode my other distance partner in competitive rides till I learned enough to ride endurance and by the time we started to do well he was 21. I am not a newbie who doesn't know how to start slow so with that said she has tyed up on numerous occasions. Usually within 10 to 20 minutes of even light exercise such as trotting on a longe line etc. I have tried this mare on Selenium supplement and had limited success. Finished one fairly tough 25 miler on her and did fine. I am wondering if there is any future for this mare who seemed to have so much potential when I bought her. Any ideas? Email links,  or post an answer as I wonder if I should continue to keep trying with this mare? I rode this mare for over an hour in 95 plus degree weather the day I bought her and had not a problem. She was in Nebraska at the time and we are from Illinois. Feed related changes? Any problems with alfalfa/grass hay mix being linked to this? Please help as another endurance horse isn't in my pocketbooks future! Should I sell this mare? Also for grain I feed a plain grain mix of rolled oats, rolled barley and cracked corn and have free choice mineral and salt available.