Why does my car keeps cutting off when I'm driving?

Yes a few seconds later.

This happens to me I will be drivig at any speed then the van starts to vibrate runs shakey loses power and I have minutes to pull over to a safe place however I need to wait two hours before my van will start/run a short distance again. Sometimes it will start right away but immediatley the shut off I have been told by several mechanics, fuel injector dirty add cleaner and higher grade gas, it's the distrubtor needs to be replaced, the knock stessor(but doesn't need replacing)i dod not know who to believe and what real repair needs to be done. The van runs like a champ when this is not happening.I do not seem to beable to drive more than an hour and it happens. Sometimes sooner please help.