I've washed my motor a hundred times.... should not cause that issue.
First. dont discount the possibility there were other issues from the prior owner and this just showed up.
Next, think of what ONE thing would cause this...I'd want to check for major harnesses, perhaps the ground strap to the motor. Also, check the alternator belt, tension and connections.
m5board dot com.... by far the best M5 resource in the world
Keep it as simple as possible...but no simpler.