tracking issue

First, I am still learning, but something I have noticed bothers me. I
have a permanent equatorial mount setup for my 8" Orion scope, polar
aligned using the drift method. I consider the performance of my
mount/drive to be excellent considering the equipment I use. However,
there seems to be a repeatable difference in the tracking error between
planets and deep sky objects.

I expect the movement of the planets is different than that of
background objects due to the plants' orbiting motion. For
observations made at 150X, should I be able to notice this difference
during a 15 minute period or is something else going on? I run a single
axis drive on the R.A. It seems that planets require a faster drive
speed with no noticed error in declination. I am certain the
scope/mount is balanced. The mirror/tube assembly is ambient
temperature so I wouldn't expect this to be caused by temp changes. I
don't have any way to test for drive RPM irregularities.

I realize there are many more variables, but I'd like to rule this
one out if possible.

my astronomy page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html