|I would say it's a coincidence, as you've already stated, changing a user account should not affect the network. |
As jackbomb pointed out, many SOHO routers are cheap pieces of BS that need to be rebooted alot. Especially if it's an all in one, Switch/Router/Wireless combination thing - I always seem to have problems with them. My guess is, crappy software causing memory leaks, builds up over time, eventually the router runs low on network buffer space & starts running like crap until you reboot it to 'free' the space.
In a real mission critical router, this would be a big deal, but for home/residential customers...they don't care, and making you reboot your router is a cheaper fix (for them) than writing proper software.
That being said - upgrade your router firmware, it may be out of date. And try rebooting it - does that fix the problem?