Every router I have ever owned or used for long periods of time has needed a restart from time to time. I considered setting up an arduino to send a ping every 60 seconds or so and have it reset the power to the modem and router if the response wasn't speedy enough. Never got around to it.That sounds reasonable but this one was stable, stable, stable for something like 2-3 years (a Cisco n-type router) and then started taking coffee breaks once a week or so.
My current router and modem combo is stable, but still needs a restart every couple of weeks.
I was in a similar boat. My router/modem would need a reboot every month or so, but nothing obtrusive. Over the past 6 months or so it was becoming an every-other-day or so issue and a real PITA. Especially because neither my router or modem had a physical on/off switch so I had to unplug them each time.
I've probably tripled my number of devices running WiFi in the past year or two, and I think that was probably contributing. I was having other issues with certain devices randomly having connection issues when other devices were working just fine. I'm not sure what the connection limit for my old router was (an older Netgear that had worked well for years), but I'm guessing that I was pushing it.
Since my cable company was starting to get aggressive in pushing us to upgrade to a DOCSIS 3.0 modem, I figured I'd splurge for a new router as well. My old router didn't support Gigabit Ethernet, either, so I'm getting a bit of a boost in performance in my wired connections as well.