Here are two ways it happens to me:
1. Create a new html doc and clear out the auto-populated content so the code window is blank:

2, Hit the Enter key once. Behavior is as expected:

3. Hit the Enter key again. Behavior is as expected:

4. NOW hit the Enter key again. DW jumps two lines on a single stroke:

5. The Enter key now produces two returns for every single stroke of the Enter key:

Of course I don't sit around hitting the Enter key over and over. it also happens when I start a new file with a comment containing the filename:

Then I hit Enter once...

And twice. Note the jump down to line 4, whereas I should be at line 3.

Mine has the same behavior you're seeing under Windows 7.
Go into Help > Submit Bug Report/Feature request and either join in on an already reported bug (if it's there) or start a new one. That way the dev guys will actually see the issue.
I don't think this is an intentional behavior.