Design medium power amplifier

Hi all,
I just made a amplifier a 2.14 GHz using the transistor ATF-50189 of the Avago. I biased the device as shown in the datasheet (Vds = 4.5 V, VGS = 0.53 V) and made all the networks stabilization to to avoid oscillations obtaining
factor stability K > 1 and |det(S)| < 1 for all the frequency reaching
GAV = 14.5 dB.
The problem was when then made the transient simulation in ADS. The output signal presented a little oscillations 8O. I don' t understand where is the problem.
Someone can help me.....

1) What is the corresponding frequency sweep for K factor analysis ?

2) A power amplifier is stable in a small-signal sense can be unstable under large signals, that's why people do large signal stability analysis by looking (for example) at the poles and zeros of the transfert function of the circuit, this is more difficult than obtaining K factor...
Try to make it stable (with transient simulation) by introducing losses in the input matching network.

I obtained this with S-parameters simulations:

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and this is transient simulation:

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Your s-parameters look strange.There are some discontinuties somewhere.
K-Factor is not sufficient standalone to check the stability.

The amplifier is oscillating around 1.2GHz, at the beginning you should recuce gain for this frequency to improve stability.

1st stage, gain=10dB, and make NF as low as possible.