e=10;l=50;
modell:={x'[t]==y[t]-e (x[t]^3/3-x[t]),
         y'[t]==-x[t],
         x[0]==2,
         y[0]==0 };
loesung=NDSolve[modell,{x,y},{t,0,l},MaxSteps->3000];
abb=ParametricPlot[{x[t],y[t]}/.loesung,{t,l/2,l},ImageSize->{400,300},
     PlotRange->All,PlotPoints->100,AxesLabel->{"x","y"}];