What does your hardware look like?
Are you doing the current sensing/control with the PIC itself (ie sampling the actual current using the ADC) (no you're not if you're using the 16F84) or are you using external comparator and flip/flops to sense and regulate the current?

When microstepping you're not only controling the currents direction thru the coil (like you are when fullstepping) but also the amplitude. If you draw the waveform of fullstepping mode out on paper you're going to get something that, with a little bit of imagination, is looking like two sine waves with a 90° phase shift between A and B. They are very crude sinewaves with only 2 "steps" to them but never the less. Now draw the waveform for halfstepping out on paper and you'll see that it looks a Little bit more like two sinewaves, still very crude sinewaves with only 3 "steps" to them but still..... What microstepping does is adding more "steps" to the sinewaves. The more "steps" you add the sinewaves the smoother the motion from the motor becomes.

So, you need to be able to regulate the amplitude of the current thru each of the two coils in a SIN/COS relation to each other.

/Henrik.