Y would van stall while driving but not during idle. Then wont start

So for sure thats the issue? Cuz im only getting 12 volts Is there anything else cuz the volt regulator is brand new according to guy who owned it b4 me is there ne chance he could have hooked something up wrong on it that would cause that problem

Alternator is brand new guy i talked to said it maybe the exciter wire. He was busy tho so how do i go about checkn that?