I don’t know what model of drain current (IDS) your using, but if you are using the Curtice model you will have a problem of convergence, specially at values of VGS<=Vp (pinch off voltage). This is due because the equation that model the drain curre (IDS) is not cero at values of VGS<=Vp . I recommend you to do a DC analysis at values of VGS=<Vp. If IDS of your model is aprox cero, for VGS=<Vp, then your model is working well (at pinch off) and the problem is not there, but if the model do bizarre thinks like negative values of IDS or large values of IDS for VGS=<Vp, then you know that the problem is there and you will have problems of convergence, so, for a transient analysis you will need to decrease the input power, by doing this you will limit the swing of the input signal to values of VGS --> Vp<=VGS<=0.
Sorry for my English OK, is not good i know LOL.