2SC4367 has a typical transit frequency of 1 GHz (min. 600 MHz), so you're lucky if the circuit is oscillating at 900 MHz at all. You all want to use transistor with GHz bandwidth instead.
Apart from this point, what's the value of C2? Is your circuit layout suited for 900 MHz at all?
If you connected an antenna, it's frequency dependent radiation impedance may be sufficient to reduce the oscillator level with rising frequency or even stop oscillations. Similarly your measurement device (was is it) may have the same effect.
B.t.w.: Are you reporting real measurements or a simulation?