The most common error when setting up a loop is to make the animation 1 frame too long. IOW if your first and last keyframe have exactly the same value so the the animation returns to exactly the same spot you should trim the comp length by one frame.
The easiest way to do this is to move the CTI to the last keyframe in the animation, then step back one frame using the page up key, then set the out point of the work area by pressing the N key, then trim the composition to the work area. You can then drop this composition in the main comp and setup a loop with Time Remapping and a loop expression or render the comp, import the footage and set up your loop with footage interpretation.
I hope this helps. If the bump is somewhere else in the timeline then you have other problems.
Thanks Rick, the bump is definitely at the end of the loop, that much I can tell, and I also think I understand the tip you just gave me...
In my infinite ignorance, I think of it as something like mending a film with scotch tape and the bump happens where the joint is and by trimming one frame we would avoid the overlap, right?
Only problem is that it still hasn't fixed it but that's probably because I've done something wrong along the process...you see, I've got no formal training with AE whatsoever and this is not the type of software you can 'click along' the learning curve...I'll keep on trying though and let you know if I succeed!
Thanks for your help