Hi,
I think you are hitting the bottom of the frequency range for that oscillator speed, try changing to 4Mhz x-tal.

EDIT: Just checked the manual....with a 20Mhz clock the lowest HPWM frequency is stated as 1221Hz for the 14bit core PICs - isn't the F88 a 14bit PIC? Don't know why you'd get 2.2kHz.....

/Henrik.