28.5C = 83.3F
29.9C = 85.8F

89F=31.6C

The temps are a little to a lot low for BP eggs, the two degree fluctuation in temperature is the least of your worries now. You need to raise the temperature to around 89F and then try to maintain a steady temperature. An incubator should maintain a +/- 0.5F or better without much problem with a decent proportional controller.

I have no experience with BP eggs incubating at 83-85F so I don't know if they will hatch or die. The last few degrees may be very important. Someone else may know the answer, I know the recommended temperature is around 89F.