Tp-link router slow

I pay for 250 Mbps internet from comcast, and I have a tp-link ac1200 router, which I have had for about 4 years. I was noticing that my internet speeds were slow, somewhere in the 10-50Mbps range using speed test even over wired. So I contacted Comcast. They had me plug directly into the modem, and then I was able to get 250 Mbps. After I had had the router off for a bit, and started using it, my speeds were back to normal at 250, but in a few days, they were down, and now I am somewhere within 50-150. Is there some look up table filling up on the router? Should I flash open source firmware onto it, or just restart it every so often? Is there a better router branch that wouldn't go bad, even after many years of use?