Your graph is interesting.
IMO, the cause of Delta Tair variations is the noise due to the turbulent rising of the cooling air inside the acrylic box. This air flows over areas of the reactor surface which are at different temperatures, so its vorticity can cause the air to heat not uniformly. The Delta Tsurf is roughly proportional to the Tmax of the reactor body whose heating and cooling trends are substantially delayed by the large mass of the reactor metal. You can see in your first graph that amplitude increases at the power-on and decreases at the power-off following trends very similar to the heating and cooling phases of the reactor.
You may be right the the cause is related to the turbulent air, but realize that the data points in the spreadsheet are 25 seconds apart. It is hard to imagine a vortex that lasts 25 seconds or longer and makes the average 2 deg C hotter or colder than the previous 25 sec.
Maybe the problem is the way this spreadsheet was derived from the raw HP data logger measurements. The logger probably sampled much more frequently than once per 25 seconds. I was assuming that each 25 second spreadsheet row is an average of the last 25 seconds, but maybe the spreadsheet just takes one raw measurements from each 25 second period. That may show the large variation we see, while averaging them (integrating over the entire period) would smooth everything out.