Hi Jordi,
I apologize for the late reply. With nonlinear simulation, it is unfortunately very hard to predict what could be causing the issue. It is possible that the high amplitude is causing an instability but at the same time it could also be other parameters. Have you had a chance to look at the following article: Nonlinear and gain methodology? If not, have a look and make sure you are accounting for the extra considerations in nonlinear simulations. Also, any chance you are using a script to setup the simulation that you can share here?
Regards,
Amrita
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