I would look at putting in more soffits vents first, then the roof vents and leave the gable end vents as the last resort. I have been reading recently, that they work the least best (least best??)


I would also have you consider one more item before proceeding.
That is: do you have an unvested gas heater, unvested gas fireplace or gas logs anywhere in your house? If so, this may actually be the cause of the problem and not lack of ventilation.
Unvested gas appliances such as those I mentioned give off lots of CO2 and H2O.
This dramatically increases the moisture content of air in your house and is most noticeable as condensation in your cold attic as frost on nail heads, wet insulation, mold buildup, etc...
Since this would not be apparent now in the summer, the stains you see could be the result of cold weather condensation. In which case the answer is not more ventilation, but rather less humidity by either eliminating the unvested heaters or adding a dehumidifier to your attic.