You said your fecal and bloodwork came back normal, but did you test during one of the "squirts" episodes or when the horse was normal, have you retested since then or did you just do the one time?
Has your horse recently been exposed to the stress of a new pasture buddy, or trailering, or even rapid changes in the weather? Anything like this may cause his immunity to dip and then you see the symptoms you're describing.
What's his baseline temperature? Have you noticed any fluctuations in his body temp during or after these episodes?
Look up info on Salmonellosis, someone mentioned Potomac fever, or your horse may have a virus.
Hope these ideas help and good luck! Hopefully your vet can come up with some other suggestions too!