Intermittent slow upload speeds

For starters, I'm on Gigablast & have been enjoying the really fast downloads, especially while being told to work from home for the last 1.5 years.

During this time, at least once a day (usually around 10a-11a) my internet upload speeds drop down from the normal ~35 Mbps to roughly .5-5 Mbps (per Speedtest.net).  This is not great when working remotely on a PC located in my office.  The mouse movement gets so laggy it's almost unusable.  

This normally lasts 30-60min, and only rarely longer than that, which is part of the problem.

Every time I call support, they tell me to reboot the router.  So, after holding on the phone & then waiting for the router to reboot, this normally has allowed the requisite time to elapse so that the problem goes away.  Unfortunately, to the tech, this means that the reboot "solved" the problem, which it did not.

So, my question, any ideas on how I can get resolution to this issue?  I'm mostly back in the office these days, but every time I work from home again (like today) I'm stuck dealing with this issue.