To get "N" speeds, both ends must support Wireless N. If your PC or Laptop only has a G wireless card, the best you'll do is G. N uses two channels and G only one. I've heard it said that just having 1 G device on your network will make everything run at G only speeds.
I use a free utility called inSIDDer, inSSIDer – Discover The Wi-Fi Around You | MetaGeek
It will give you a nice view of whats going on and whats on what channel(s). It sounds like your Router has issues, maybe interference from another device. My cordless phones would throw my DIR-615's WIFI off when they rang. I ended up replacing the phones with dict 6 phones.
The reason I asked if you changed your SSID was to make sure you weren't using the same SSID as another network. That could cause problems, maybe knocking you both off.