Have you considered a high mount turbo?
Have you checked the O-Rings on your fuel fill? Water can get into your fuel that way and create your problem.
No you do not have to drain the oil.
The CHARLES pictured is indeed a voltage regulator. Inside the box there are two regulators. You might try switching the wiring to see if the problem moves to the other engine. I know that Charles no longer makes that double regulator but if you find that that is the problem call them as I have found them easy to work with and they can probably repair it.