Getting half my internet speed after rebooting modem (speed fluctuates after each reboot) - MOTOROLA

I'm paying for gigabit speed and usually my speeds with Xfinity are fantastic, close to the advertised 1000 mbps (usually in the 900s in the speed tests).    Today I was doing some work on the power outlets in my house and turned off my MOTOROLA MB8600 modem and turned it back on.   When I did so, I noticed my internet running unusually slow so I did a speed test and I was getting 24 mbps!!!   I unplugged the connection from my router, connected it directly to my laptop, rebooted the modem and same results. 

After a couple of more power-cycles of my modem, the speed tests would fluctuate between 24 mbps and 450 mbps.   The 450 was the highest I ever got back again.  I think that every time I reboot the modem, Xfinity is putting me in the wrong service tier or something, but never back to the correct one.   I can't explain what else could be happening. 

Has anyone experienced this before?  I want the speeds I pay for back! 

Thanks in advance.