Oscilation

I'm doing a design of a amplicador (2.4Ghz). I have at 2.4Ghz S21=10db s11=s22=-40db. But at 500Mhz, I have s21=8.433, s11=-1.092 and s22=-3.139.
This is a potential oscillations?How can I save the problem?

Stabilitty is not only limited by the parameters that you mentioned above but also on S12 which is missing.

Up to 5GHz the circuit is stable.
stability is defined by K and delta but currently only one parameter is sufficient to confirm the stability and is Mu.
If Mu is >1 then the amplifier is stable which is present in your case.

You are using Low Pass matching topology for both, input and output.
Use a High Pass matching network at the input and you will minimize the gain at 500MHz.