Conflicting paths

[Richmond]

I have a silly solar system with 2 planets and a stationary sun.

Now in my "Do it" button I have this script:

on mouseUp
    move img "Saturn" to the points of grc "OVOID" in 25 seconds without 
waiting
    move img "Mars" to the points of grc "PATH2" in 25 seconds without 
waiting
end mouseUp

now img "Saturns" trots smoothly round my rather jagged, freehand oval,

BUT img "Mars" jerks and pauses like an alcoholic on a major binge-drink.

Obviously that is not a good thing [unless one wants to produce 
animations of alcoholics],
so would be grateful if somebody could tell me where I've slipped up.

Richmond.