likely too simple to be a factor but on the off chance have you checked recently to see if someone has setup a wifi channel that overlaps yours? my sister had similar issues went & scanned channels and found the default channel her modem picked was also the same as several of her neighbors. one so powerful it was only 3-5dbm difference.
simple to change channels once you know which is best or worst. i did that for her and she's been good since. i didn't want to deal with it anymore, for her, so i gave her a google wifi system at Christmas which picks the best channel and changes as conditions warrant. again you likely know all this but never hurts to put the info out there. hopefully you've got it resolved by now.