I was operating my Dewalt 618 router recently and had the speed set to around 4. In mid pass the router got much louder and higher pitched and I realizes that it sped up to full speed. So I appear to have lost the variable speed control. Is this a common way for routers to malfunction? Is it typically an isolated thing that is easily repaired or does this signal the end of the router's life? I haven't used the router much over the 5-6 years that I've owned it.