Hi,
I also have the DB and have temp swing of around 50f. There is lots of talk on pelletheads.com about temp swings and it affects all pellet grills to some extent. I too have had a controller replaced but i think they're on really old firmware being for the UK market they don't get updated.
I think it's designed to hold the set temp when it's loaded with food and i've noticed the swings are less when more food is in it. The main thing is to learn your grill. I bought some cheap oven type thermometers and measured the grate temps against the controller temp. i found out the right side was hotter and also front to back the back was hotter.
I ended up just using a probe in the centre of the grate and adjusting the controller to match the probe. i also put a dome thermometer in the lid for fun and it often matches the probe. playing with the baffles can change thing, some people don't use them.
The main thing is, i get excellent food. All the low and slow is great and using the grill as a wood fired oven for high heat chicken. I have no complaints
On start let it get to 70c first before you set your desired temp. Less overshoots that way.