What happened to the accuracy before and after 75th epoch. Why its unstable at first, then stepped up after the 75th epoch?
I think that it is a convergence of the gradient descent happened. While training,  we are looking for correct weights and that is why we see these fluctuations on the picture. When we are close to a global minimum (found good weights), the model starting to converge. That is what happened after the 75th epoch in this case.

By the way, you have an overfitting