I've had the same issues with my 340. Turns out if I let a little too much residue (ash, etc) build up in the burn chamber it will start doing that. Once I vacuumed out the excess it went right back to staying right on temp. Now that I keep it clean it works well.
This post is a bit old, but I thought I'd add my recent experience. I've had my 340 for a couple of months now and have been pleased with how the temp seems to stay quite close to the set temp.
Late yesterday afternoon I put on a nice looking Boston Butt (7.69#) at 200*. The first thing I...