Connect the buzzer directly.
Hi,
You can directly connect a buzzer to a PIC I/O. However it is a good practice to have some sort of current limiting resistor in the line (say around 200 ohms). This prevents the PIC getting fried in case of accidental short-circuits in the output.
Buzzers generally work best in the mid-high freq. range. In the higher and lower frequency the outputs will vary. Also do not expect a very high volume with a 0-5v output toggle.
I did not understand what you mean by "not stable"
Regards
Sougata