One question I have is, How do you have the pressure sensor mounted? If it mounted in such way that the thrust is being impressed on the sensing element then the pressure readings will be offset. I would mount the sensor so that the sensing element is perpindicular to the rockets thrust. It should give you a much more stable and reliable reading. I would also take about 64 readings and do a right shift 4 places to the give you a 12 bit reading. I do this as a mater of fact for all of my A/D readings from the PIC internal A/D. I only use a 40uS. delay when changing channels. You should easily be able to captute the data at 100 Hz.