Netgear router suddenly slow

I have a 4 year old Netgear RT314 router which has worked perfectly until now. My internet download speed recently dropped from normal 3 megabits to between 4 to 100 kilobits, intermittently. At times normal, at times very slow. I thought it was my ISP until I connected the PC directly to the cable modem. Everything then ran normally, all the time. As soon as I put the router back in, everything slowed again. I reinstalled the router firmware and everything again worked normally, for two days. Then it was back to slow slow. Took out the router and everything back to normal. What is going on and what can I do about it? Is this router dying? Can it be saved? Thank you stvwlf