Hi
First of all I have no idea why you have that happening.
However it might be useful if you could say what format of clip you are using and it's size in pixels. Also, when you say "replaying" do you mean previewing the clip in the right hand screen?
Is there anything odd with your sequence if you switch into storyboard mode. Assuming you have inserted two clips, you should just have two thumbnails of your clips with a square between them with 0.0 in it and each clip should have its duration shown in a box beneath each thumbnail.
Do you still get the effect if...(This time in timeline mode...)
You drag clip 1 to the sequence and then drag clip 2 to the sequence so it follows clip 1.
Now move the red cursor line to the middle of clip 1 and split the clip. Now drag clip 2 back to the split (You will see a pink arrow appear to let you know where to release it)
Drop it in between the two halves of clip 1. Now, when you play it in preview, do you still get the stutter as the new joint is played over?
Nat