The major factor causing the drift is the power supply voltage. Whilst developing the beacon using just the Gadget Gangster board I was powering it with a 6V supply - the minimum needed. After adding a PA I needed to increase the supply voltage to 9V. The drift is greater with 9V, regardless of whether the PA is actually connected.
|Looking down on the Propeller board|
Whilst it is useful to know what is causing the drift, my discovery has not indicated an easy solution. Either I use a temperature compensated 10MHz crystal oscillator as suggested by one of my readers, or I use the Prop to control an outboard and more stable synthesized oscillator. The first option looks the easiest, but the TCXO is not an inexpensive component.