It's likely due to parasitic capacitance, but could possibly be due to temperature change as well. Try blowing on the heatsink and see if it produces the same drift.
If it's capacitance, then the best solution may be to put a thin RF screen between the transistor and the heatsink (with insulation on both sides), and connect the screen to circuit ground. This will detune the circuit somewhat, but the effect will be much more consistent and you can adjust the tuning back where you want.