The stock oscillators are crap, it'll drift more than the SS Minnow
They're also quite difficult to adjust with any precision as Dave discovered - especially if there is any slack in the trimmer. Also, sometimes, any change can perturb the oscillator by orders of magnitude more than the adjustment you're trying to achieve.
I'm surprised that the oscillator isn't voltage controlled with a DAC to generate the V
FC - that is not only a better way of doing it but lends itself to automatic calibration rather more easily than sticking a screwdriver into a trimmer. Especially as the tongue angle calibration on most units tends to be very poor
I suspect that these were intended to be used with the Rubidium standard rather than the inbuilt oscillator.