If you can figure out exactly when these episodes are happening that might help. I don't really know what else you can do. Regumate or mare plus are the only mare supplements i know of. You might want to check out the arab section here or reproduction and ask for lois. She knows a great deal about this stuff and could help you out a lot more than i can.
Horses are unpredictable.
If you are not experienced, handle horses under the supervision of a qualified instructor. This forum is not intended to be an instructional guide, and any one who uses it for this purpose, does so at their own risk.