Thanks for the info on heat sinks, SarahIrl! I've got one in my incubator, and when it was running empty I did notice that the heat sink helped stabilize the temperatures. I couldn't find a small enough water bottle, so I used a Ziploc sandwich bag about 1/3 full of water, rolled and taped...
You've got my sympathies, Orange Ribbon. I am having similar barrels of fun with my Brower Top Hatch. The dang thing's got a gremlin. It will hold steady around 99.5, plus or minus two degrees, for days. Then it will completely lose its mind and drop six degrees, then shoot up six degrees...