Sounds like you need to do some PWM, filter it with an RC network, and if you need to pump this into a low impedance load, use an opamp to buffer the output (Check out the PWM command in your PicBasic Pro Manual).
Here's a link to something that might interest you:LED fade without PWM command
However if you need the ability to do other things while the pic is maintaining the PWM output, you would be better off changing over to a PICF628 instead which has a hardware PWM output. Using this pic with PBP's HPWM command, you basically have a set it and forget it situation, with the pic handling the PWM in the background. Check out this link for more info on this chip and its built-in PWM capabilities: http://www.rentron.com/PIC16F628.htm
I hope that helps you out,
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