10W 900MHz PA design

I want to use Renesas MOSFET NE5511279a for 900MHz application.
Problem appear even at small signal Gain, and DC characteristic.

1. conjugate match at RF Input, output only give 6 dB gain, at least 10dB less compare to spec. (set Drain current @400mA, Vgs=1.8V, VDS=7V)
VG is biased by 4.7k resistor for current limit, coil choke at output.

2. If i fix the VDS at 7V, and increase the VGS from 0V, I can get Vth at around 1.5V, 400mA@1.8V, 1A@2V, but... further increase in VGS>2V does not show any current gain.

3. It is observed that when the transistor is getting heat up, iG would increase, causing gate voltage drop. Is it a common behaviour to MOSFET?

4. To void this, it was tested at 10% duty cycle, but max gain I can get is still 6dB.

Pls help

For me it looks fine.
Check the matching circuit, connector interface loss, calibration with the cables etc.