I think you have a couple of different symptoms going on.
The seizure are most likely due to the lack of calcium an I'd suggest liquid calcium to get him back on track. Unless you possibly have a beardie with neurological problems. I'm sorry I didn't' read anywhere and sorry if I missed it but how old is this beardie? I mean baby? Adult? Not all beardies have the same hardiness...some may have weaker systems this may be the case with this beardie sense the others don't have issues. Plus not all beardies absorb calcium as well as others. And without the proper levels of UBV and temps many can not.
But my first thought when reading this thread was that your beardie sounds like it has a respiratory infection also.
He needs to be in his own enclosure not with others...you wouldn't want others to contract it. No, the symptoms can come and go depending on how they are feeling...just like when we get one. Sometimes we feel better and start moving around then we tire out and the symptoms return.
the warm end of my beardies tank is about 102 degrees F and the cool end is about 78 degrees F
What is the basking temp directly on the basking spot? Also having your UVB that far away isn't very beneficial to them....over time they will become ill because inorder for the immune system to function properly they need the right amount and levels of UVB.
Zak-n-Wheezie tend to get URI's on a regular basis due to there condition because they are in a constant form of stress...stress compromises immune systems. What you described sounds like what I see with them. Tom is very correct about the phlem and vomiting. More than likely if there were not food in his belly when he vomited then you would have seen a foamy phlem come up. Does this beardie tend to put his nose up in the air or rest his head with nose pointing upward?