Suburban hot water heater surging

[tperschb]

I have a 2017 Minnie Winnie 26a with a suburban sw6d water heater. After about 12 minutes of heating the water it is very warm but not hot and then water surges out of the relief value. I have tried numerous methods for an air gap with no relief. I shut off the cold fill value to stop the water surging.

[Ray,IN]

To re-establish the air pocket that must be in the top of the water heater, shut off cold water supply, open both hot and cold faucets to bleed off all pressure, then open the over-pressure valve lever on the WH and allow water to drain until it stops. Let lever on over-pressure valve snap closed. Turn on cold water supply, and its done. This procedure may have to be performed several times a year, as water naturally absorbs air under pressure.

[Morich]

Water surges seems to be a bit odd description so maybe there is something else involved?
It may be good to keep in mind that the pressure relief valve may be getting to the point of no holding pressure as it should and needed to be replaced?

Possibly take a close look at spaces where a water leak might be letting the air gap fade out and then the water leak not be noticed?