A P-1db result with a strange plot, why?

I am doing P-1db simulation with Cadence.
But the waveform is quite strange with a sudden and sharp decrease in the line.
Why dose this happen?
This phenomenom happens when some device parameters vary just a little bit!
THanks.

It seems that, for that input power, the simulation has problem to converge. Check output log to confirm.
I hope it will help.
Mazz

At that point, there should be convergence problem, that's why result is weird..

Thanks Mazz and Bigboss.
You are right.
And I have read the post PSS dosen't converge with nport component.
For my case, there is just a resistive nport load.
It's strange that it failed to converge when I just change some parameters very little bit.
And with DFT, I once used it, but as it requires the same initial state and end state, I think it is hard to choose the DFT window.

THanks again.