Why interrupts? -> Can't get 1kHz steps on "high" frequencies: Ex. 201kHz then 202kHz..
Why not use PWM? -> Same as above...and also can't get low frequencies like 1Hz, 2Hz
Why not use CCP1, CCP2?
One of the PWM examples shows changing the dutycycle by 25nS. This gives me the idea you could change the frequency in 25nS steps.

Where is this example?

Thanks again.