What is the treatment for that? It. ?

My father's age is 79.But still he is healthy and fit. But one problem of his body give us tention. Sometimes he becomes senseless.Then he stays in a black out condition for 5-10 minutes. He becomes fit after that automatically. But then his pressure stays low. Actually his pressure is always low but stays in a rance. But after that his pressure becomes abnormal (90/60 like this) for a while. This happens once or twice in a year . So we are too much worried about that. We visited heart specialist,Nerve specialist, etc etc doctors and did various tests. His heart is ok and all things are ok by the reports. This is continuing near about 15 years before. So tell me please what is the disease and what is the treatment for it ...???