Performing a robust fft of a sinus

Hi,

I want to perform a fourier transform near the Nyquist frequency. I plot in a loop the maximum of the FFT for an increasing length of the sinus.

However the result is really unstable, regarding to both the length of the sinus and the sinus frequency.

 Fe = 400;     %sampling frequency, Hz
 F = 163;      %sinus frequency, Hz
 A = 26;       %amplitude

 for i=10 : 100
    clear sinus, clear vect_temp, clear FFT, clear FFT_prime;

    N = i*100;
    vect_temp = (1/Fe:1/Fe:N/Fe);  %s
    sinus = A*sin(2*pi*F*vect_temp);

    FFT = abs(fft(sinus))/ length(sinus); %FFT normalized

    NFFT = 2^nextpow2(length(sinus));
    FFT_prime = abs(fft(sinus,NFFT)) / length(sinus); %FFT normalized

    hold on,
    plot(N,max(FFT),'+');
    plot(N,max(FFT_prime),'*r');

 end

Any idea what's the problem?