If you really need fine control two 555 chips will give you complete control with a couple pots. I do not have experience with these myself, but run one in astable mode (google this) to generate a square wave with variable frequency (assuming you have one pot here), then the second can control the PWM of that square wave.

I'll try to remember to look at work tomorrow, but there was a similar chip I saw used in such a circuit. These can be obtained in 8-pin DIP packages, so would be easy to work with.