Pretty much any transistor that has high gain (like 15 dB) at microwave frequency is going to have a HUGE amount of gain at low RF frequencies (like 30 to 50 dB). It is that excess gain that causes the instability.
You can use filtering mechanisms in the base and collector bias structures to terminate the base and collector in the right impedance at these lower frequencies without screwing up the microwave operation too much. Also, since your problem is at low frequencies, sometimes it is easy to apply some negative feedback at the lower frequencies to help to kill that excess gain.