Gigabit speed being throttled down

If you're having the issue on multiple routers, then it tends to point at either the line coming in, the modem, or some way you're setting up the routers. 

Comcast technicians aren't the best for phone support. they're main goal is to get you off the line. 

Do you have a screen snip of all your modem connections info? This can help a lot. As well as the logs. 

If you have any splitter, attenuators, amplifiers in line, I'd recommend removing them. Preferably, I'd also recommend moving the modem to right where the coax enters the home. If that isn't possible, inspect the line for any damage, kinks, cheap/poor connectors, loose connectors, etc. Even small things can cause issues when you're running gigabit. 

You've put that its happening every 1-3 hours roughly. If you hardwire into your modem for that period, what speeds do you get? 

Are you using any port aggregation features?