Play around with the density settings/ steps. Other than that your super simple shape could easily be drawn from scratch without even using the extrude function, resulting in a much lighter file to work with and avoiding all the issues.
Which version is it?
Is that with or without GPU rendering? And what happens if you use Preview on CPU?
Did you already expand the appearence?
Did you apply artwork to the 3D object?
Can you perhaps upload the AI file?
I use the current updated version of AI.
I'm not familiar with GPU rendering. Maybe you could help with that?
I did expand the appearance.
How to apply artwork to the 3D object?
The file: Not that one but here is another one.
I notice the lines pop up as soon as I click on gradient.
Sorry I could not get the AI file to upload.
OK. The 3D effect creates a lot of single objects. When you apply a gradient to them, it will sit in each of those shapes.
You can see the shapes when you go to View > Outline view.
So what can I do about this to get a smooth gradient?