Such a problem could have several causes. At first, however, I would have guessed its due to a weak or unstable network connection too. If your workaround with the extender does not help, I would recommend connecting the Pi directly to the router via Ethernet (just to test whether the problem then no longer occurs).
If that doesn’t help, I could also imagine that the SD card slowly goes belly up. How long have you been using the Pi with the SD card? Do you have the possibility to burn a fresh image onto a (possibly new) SD card?