Just upgraded to 1 Gig internet, but having issues with download speed.

As the title says, we upgraded to 1 Gig download speed today. Problem is, we aren't getting anywhere near that and the speed is not consistent.

My modem and router both support over 1 Gig speeds, so that's not the issue.

I've been testing all evening and the speeds are all over the place. As low as 540 and as high as 700.

My ISP said we should be getting at least 800 consistently. Obviously, that's not happening.

Is 1 Gig internet known to be that unreliable or should I have them come check the lines?