Defective unit?

I just got a Anova Precision Cooker WIFI as a gift last night, although after some initial use, I am curious if I have a defective unit. This is my first sous vide device, so I am not entirely sure if there is some adjustments to my method that need to be done, or if the unit needs replacement.

I started with 10L of water at around room temperature in a clear polycarbonate container. I covered the top in saran wrap. It took 2 hours for the water get to a temperature of 75C. During the heating process, I could see a slow increase temperature, and then it would drop a couple of degrees, go back up, drop go back up and so forth.

Did a little more experimenting and I think the poor performance might have been from the outlet it was plugged into. Tried a different outlet in the kitchen and performance seems to be better and without all the dips. Will experiment with it a bit more to see if that is the case.