you will certainly get severe vertical elongation near the horizon due to differential dispersion smearing the image - it is independent of tracking and will be there even in very short exposures, as Alistair was getting at.
test if that is what you are seeing by using a narrow band filter (Ha would be best) and seeing if it is still there - if dispersion, it should be very much reduced
ref:
http://www.paquettefamily.ca/astro/star_study/