Only wifi. I've no devices with an Ethernet port. It may be worth borrowing one to confirm for sure that the issues are wi-fi. I agree with the other posts: everything you say points that way; but all extra data is worth while.
It may be worth trying to move the router somewhat. Even quite small movements can make big differences to wifi if there is masking or interference. If possible (depending what devices you have) try installing something like inSSIDer to check signal strengths and channels.