Several things can cause that issue. DVR, ONT, signal, etc..You can pull up hidden diagnostics on your set top box/dvr to see what the signal is like coming in from the ont to the box. That would be a start. If its low that could be the issue. If not it at least you can rule that out and could be the ont itself is bad, main line, etc..
You'd have to google how to get to it on the model of your box since its different for a few of them for pulling up the hidden menu.
I'm sure others will eventually chime in.
I have recently had the exact same problem on a couple of occasions. I have no suggestions how to fix it.