(My new DB's power cord is incredibly cheaply made. The DC connector is very loose in the connection to the power supply box, to the point that if you just pick it up to move it, the grill immediately loses power. The small connection point on the tip doesn't stay out like it is supposed to, so I think the spring is broken inside. Could that be your power issue? )
I too just picked up a DB this last Friday. I have a few friends who couldn't recommend them enough. So far it's done ok with a few exceptions. Like you, I get some pretty significant temperature swings, especially in the 200-300 degree set cooking range. If I set at 225, it will cruise up to 250 fairly often. I had it at 200 and was hitting 230 from time to time, although it seemed like it would only to go 220 more often than not. This was continuing after hours of being on, and no food in it last night while I played with it.
After the initial burn in on Friday night, I did a brisket on Saturday. Brisket turned out great, but the temp swings were enough to drive me crazy. They were easily going up 30 - 40 degrees past my set temp, sometimes even 50-60 degrees higher. Had it set at 230, and would swing up to 260-270 all day long. I understand that we're talking about fire burning fuel and I expect some swings, but that was too much. It got worse as the day went along, and eventually even the amount of smoke got down to almost nothing which surprised me. Last night I cooked some chicken and vegetables, and things just didn't seem right. So after I was done and everything cooled down, I took out the grease tray and heat shield. There was so much ash build up in the firebox that you couldn't see the igniter. It was so thick I could pull it out in clumps (no unburned pellets, just ash). Keep in mind, this was only after 1 long 11 hour cook. I thought the fan was supposed to blow all of that out of the firebox? We're very dry and arid here so moisture shouldn't have been an issue and I was using GMG's pellets. I cleaned everything up and vacuumed out the firebox and internals. Smoke started rolling again after I fired it up like it should have been, and temp swings went from a solid 30-40 degrees to down to averaging about 20 degrees from set temp as I stated at the beginning of this post.
The strange thing is, when at 150, it stays within 3-5 degrees of 150 on the uphill swings. I tried it at 350 & 500, and again, it stayed within 3-5 degrees on the uphill swings. Not sure why it's working perfectly at those temps, but in that 200-300 range it goes bonkers. Which is frustrating since that's the temp range I'll do most of my cooking at.
I ran it up to 500 last night to sear the chicken I cooked, and it started smoking off the bottom (outside) of the grill and it stunk like electrical burning or plastic burning. Looked up under there and I couldn't see anything but metal. The older models had the combustion fan under there, and I've read a lot of issues with those melting from too high of heat, so they must have changed that... Looks like on the Prime Plus they are now just using the 1 fan instead of 2, unless I'm missing something? Still has me worried that something was burning or melting that shouldn't have been?
Lastly, the lid doesn't close evenly and leaves a quarter to half inch gap you can see through on the sides. Friend of mine suggested I put a 2x4 under the bottom of the lid against the grill and push down on the first bend up above to try and get the bottom to flare out since the very bottom is the only place it's touching and needs bent out (his was the same way). This part is really frustrating after spending $800 on this grill. I'm not expecting perfection or a perfect airtight seal but the lid should close all the way around. My dad has a Traeger, and the lid touches all the way around no problem.