You may be experiencing DC bias oscillation. Long test cables (i.e. test setup) may increase DC bias lead inductance making the PA unstable. Adding a large capacitance (such as 47 uF in correct polarity) across the gate voltage bias leads should help with this issue. Shortening the test leads as much as possible should also help, making the additional capacitance unnecessary.
It is a good idea to initially set the GaN PA Drain power supply current limit to just above the lower “Idq” setting so that a DC bias oscillation does not damage the PA. If all is well after initial bias, the Drain current limit can be adjusted to the full value with RF drive.