How can I improve my Freq to Voltage circuit?
I suspect a big part of the drift you're seeing is related to this equation in the LM2907 datasheet:



This shows that the LM2907 output voltage will drift linearly if your Vcc drifts. If you are not using an extremely good regulator to power the LM2907, this could easily account for 0.2% of drift.

If you are using a precision reference circuit for your ADC (or a bandgap reference internal to the uC), you might try switching to using Vcc as the reference. With a Vcc-referenced ADC any affect of Vcc drift on the LM2907 output will be compensated by the drift in the ADC operation.  

Edit: I remove the prior advice because I see you are powering the LM2907 from 9 V, while the PIC is powered from 5 V; therefore, you can't expect the two power supplies to drift equally.