Install Router Stats lite on a PC and set it running with 30 second updates. Get a couple of 24 hour plots showing Sync Speed and SNR and link to the images.
28dB sounds rather high for an SNR figure ...
Look at the local environment. Is there another company close by that could be the source of electronic noise? I have come across offices which had abysmal performance up until 12:00 - which was when one particular part of their factory was working.
The RSL plots may help with the story.
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