Code 58 may be bull, as alot of 92'* have that problem for some reason.
Are you sure the PCV valve was installed correctly?
How could an intake valve cause that puff?
You've already stated your plugs are oil fouled. You have oil in your cylinders. The plug tip damages a valve, so that it can't fully close. As the cylinder comes up on compression stroke, the oil is pressed up into the lower intake.
Just a theory, but it makes alot of sense. To fix it (assuming I'm right) you better be very familiar with pushrod engines and buy a maintenance manual. Depending on the mileage of the motor, most people would replace the whole motor for something like this. There may be other hidden damage, like scored cylinder walls, etc.