it could be, there are other routers nearby that are causing some interference - I would try:
- change the channel (usually 6 is default, I had to change mine recently to 1, there are over 20 APs I can see in my network scan lol)
- upgrade the router firmware
- see if any client driver & software updates are available
- if the router is compatible with DD-WRT / Tomato / OpenWRT etc. some of those allow to boost the power
Can't comment on powerline I have no experience - in my house, I was lucky to have cat5 used for phone wiring, so I was able to splice the pairs up and get 10/100 jacks on all my floors, and used that to move my AP up to the middle floor to get signal distributed more evenly.
edit - you can also try something like this for less than $5