AT&T U-verse → Service interruption

Encountered what I hope is not the beginning of ongoing problem... 2 instances in the past week, where the U-Verse gateway (3800HGV-B ?) reboots unexpectedly, resulting in loss of television and internet, not just wi-fi...complete service interruption. After about 2 to 3 minutes, the system is back and service is restored. This gateway is the only one I've had with U-Verse for 3+ years..I've not encountered this behavior before...Any ideas?...

Sounds like a similar thread that some other users were having with a 3800HGV-B. The other user ended up getting a tech to go there and just replaced his gateway with a NVG589. It's possible that the problem just lays on the gateway.
two questions tho, do you see all the lights on the gateway go out except for the power light when it happens? 2nd, do you have fiber coming to your home (data cable connected to red port on gateway) or copper? (data cable connected to green port on gateway)

If all the lights go out except the power light and if you are on copper, I would suggest trying to get a gateway replacement with a NVG589, either by getting a tech to go to your house, or talk to tech support and manage to get them to send you one. If on fiber, it could possibly still be the gateway, but it could also be the ONT or power supply that feeds it.

While possible and likely 3 year old RG may be issue.... Also likely could be line issue.
If willing either download the UVrealtime program and post screen shots and/or download the myatt app to device and go to ufix then troubleshoot.... This runs same 5 tests as when calling in for service, a green check mark means no issue found within testing parameter but problem found will assist in creating a service ticket for tech visit that could both resolve line/house problem and replace RG if needed.

Your rebooting is likely result of too many errors (inside wiring, outside wiring or foreign voltage from your equipment especially flat screen tvs), but could be power related or faulty equipment.