http://www.vdl.umn.edu/ourservices/canineneuromuscular/taylor2008/home.html
here is a link to EIC "exercise induced collapse" it does sound like what you are describing you dog attacks. Interestingly that they are using Pheno as a treatment.
There are now numerous reports of severely affected dogs ...

Have you considered Seizures? When was the last time these dogs were vacinated prior to the first episode?