Seems that you are using a different schematic than the one recommended by Freescale as a test circuit.
I don’t know how this will affect the output impedance at high output power, but I think is good to place at the output at least the LPF matching network recommended by Fresscale (Z19, C5, Z20).
The quality of the output capacitors is very important getting a rugged PA, and is good to use the high current/voltage ones recommended by Freescale.
I don’t think the small green trim cap that you use at the output can handle the high RF voltage and current.
Also check for low frequency oscillation at high output power and different load impedances, which can damage your transistors.