Accuracy noise patterns during model training
It is likely that in your training phase you are reaching local minima, then since you 'persist' at each epoch you get off of your point and then after the 800 epochs you reach it again.

Look at the image below, imagine you reach the blue point, but you keep persisting, looking for another point, then you will be off of your track but it will eventually come back. The pattern will be more clear in logistic regression since theoretically you are applying gradient descent to a simpler function than a complex neural network.