What happened to the accuracy before and after 75th epoch. Why its unstable at first, then stepped up after the 75th epoch? What happened to the accuracy before and after 75th? Why before 75th, its unstable and suddenly stepped up after that? I am using RESNET and CIFAR-10 dataset.
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