Modem loses speed from 500 megabits per second down to 92 megabits per second

Almost every day now, when I get home from work, I test my modem speed. It's at 92 megabits per second. I unplug it and plug it back in, and it's at 500 megabits per second. I go do some work in my workshop, come back in the house, check the modem, and it's at 92 megabits per second. Is the modem being throttled, or does the modem have a problem and needs to be replaced? Thanks.