802.11g router... Who is your ISP? They usually supply you with the latest Router. Just ring them up and tell them yours is broken.

The only wireless part is from router to your laptop, if you have walls etc inbetween this can block the signal making it unreliable.

The latest wireless standard is 802.11n or wireless n. Routers and modem routers that use this are speedier and give a much more stable connection than the old 54Mbps or wireless g equipment

Is your laptop fairly modern and not restricted to wireless g? I'm also presuming you have tested your Internet connection speed while laptop is connected with an ethernet cable to the router?

"On average, every couple of days I realise that my downloads and sometimes uploads have slowed. This is rectified by unplugging and replugging in my router"

Given those symptoms, a new router may solve your problem. I've installed a few of these recently and have found them excellent performers, reliable, and very reasonably priced. At under £40, it won't be the end of the world if you find it doesn't fix things.

Note that if you badger your ISP for a replacement router, it's likely they'll insist on you agreeing to stay with them for a further 12 or even 18 months.

"or sometimes by going to speednest.net checking the ping and speeds, then choosing "new server" and retesting. The retest is always back up to normal."

Now that's the bit that really puzzles me. No website, not even a speed-testing site, can have any affect on your broadband speeds so perhaps it's a different speedtest site you need - here's an alternative, reliable one you can use.

"Does this sound like a router problem or just a fluctuating and poor signal?"

If you mean the wireless signal between your router and laptop, then the next time you get a slow-down, check the laptop's reported wireless-connection speed. If it's reporting less than 2Mbps then that would be a problem. I doubt very much that's what happening to you though.