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is your C2 a high stability component - do not use ceramic!! Like wise the resistor chain must be high quality components especially the variable device. Have you logged the frequency drift with the temperature?, you could try thermal cycling it to expedite any burning in processes.
You could use a crystal clock and divide the frequency with a digital counter to get 35kHz. For example a standard 3.64MHz crystal oscillator can be divided by 104 using two 4-bit programmable counter chips to get 35kHz, which will be accurate to within the accuracy and stability of the crystal you obtain.