there are two kinds of unstability1)the rf unstability(2)the feed DC power unstability:
the resistor in the drain only conquer the feed DC unstability,but not RF unstability,try to add a resistor in the rf chain
I used a similar device a while ago and the problem was stability at lower frequencies. To overcome this, add a resistor in series with gate, bypassed by a capacitor at your amplifier frequency. Should help!
Instability not necessarily occurs in your band of intest. It can occur everywhere!
There are several ways to stabilize the transistor:
1. add a series resistor at the gate.
2. add a parallel resistor at the gate
3. series resistor at drain
4. parallel resistor at drain
5. a feedback (drain to gate) resistor with capacitor.