Tying up & cramps

["B. Miskimmin/J. Curtis" <limnos@junction.net>]

Hi folks,
I'm trying to do the best for a horse that tied up only once during
conditioning in the spring, then a few months later had minor cramps at 2
rides causing him to be pulled. He finished 2 other 50/60 mi rides just
fine at a slower pace. His muscle enzyme levels were very elevated after
the tie-up; no blood work done after the cramps at rides.

After the tie-up, I had the horse on pasture, small quantities of oats,
with supplements of selenium, Vit E and other minerals/vitamins, plus veg
oil. Found out our area is deficient in selenium. I gave him electrolytes
at the rides, but I'm not sure if the balance was right as the e-lytes are
not specifically for horses.
I think we have the serious tying up under control but my concern is, what
to do about the cramps. They set in both times while waiting for vet checks
even though I kept him moving as much as possible (once at the finish of a
very hard 50 - boo hoo!). The cramps were always in the left hind, and he
always came home after the rides and trotted away in the pasture sound as
can be.

Questions:
Is it possible for horses to overcome the tendency to cramp? How important
is the e-lyte formula (I am planning to switch to a better formulation next
year)?  
Has anyone beaten this type of situation using other supplements at rides
or otherwise? When the cramps occur in the same year as the tie up, is it
possible that those muscles have not yet recovered from the damage caused
by the tie-up? If so, does that mean I can hope for better next year?

This horse is full of energy all the time (but he's not overly nervous),
and even got 13/12 for CRI at one of the pulls above.  It seems that all
his systems are go, but I have to manage him for this unpredictable cramp
situation.

Would like to hear other's experiences and advice.
thanks,
Brenda