Ok here is what I've found so far:
Ipod 5th Gen (known as Video) Jumps
Ipod Touch 2nd Gen Jumps (happens with both touch's wee have)
Ipod Nano 2nd Gen Jumps ever so subtly. Its hardly noticeable but if the following track starts off loud then its just noticeable and no more.
Ipod Nano 4th Gen Doesn't Jump (the tracks can sometimes stick and not move on in the display though)
Ipod Nano 4th Gen Jumped for the first couple of songs then was absolutely fine (no sign of the tracks display sticking)
I can't find my ipod mini

Here is the strange thing, on the ipods that jump if I select SCAN on the RCD510, it will play the first 10 seconds of each song and and not once does it jump, yet if I play those same songs in their entirety they will jump. Which to me suggests the RCD510 rather than the ipod??
Anyone ?