First I would check that your layout is appropriate, according to the manufacturer's recommendations. A little extra source inductance can have a large impact.
You may need to add lossy elements to the amplifier to get wideband stability. This can be shunt resistance on the gate or drain to ground, or a series RC connected between the gate and drain. These methods will somewhat impact the small signal gain and noise figure of the amplifier, but this isn't usually critical for a RFPA.