This is in contrast to having ~30mbps upload stable across the whole day before upgrading to 50mbps upload.
Are lower lows/less stable upload speeds expected/normal behaviour when on a gigabit internet package (Had 500 download, 30 upload before upgrading to 1.5gbps, 50mbps)
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I can understand how frustrating it is to experience this reduction in upload speeds at certain times of the day, I am glad to hear that things are looking better.
If you do continue to experience issues, please follow the steps outlined here to locate your signal level table for your modem. Please post the results to this thread. We can review the signal levels to see if there is anything concerning before determining what steps to take next.