Thank you for your response!
I tried applying the initialization methods from ashunigion’s post from this stackoverflow post. Unfortunately, I couldn’t apply all of them to the Conv2D layers as well because they don’t have an in_features attribute. The ones that worked couldn’t fix the instability either though.
What else I tried is to play around with the momentum. Currently it seems to me that a lower momentum would be more stable. Are there any known alternatives to making SGD more stable with Nesterov without lowering the momentum itself?