the problem here is Sketchup's concept of 'order', when you first draw a set of lines, they are in order. if you edit one of more of them, the order changes. when you edit some other part of the drawing, the order may change again.

you can see this clearly by using the freehand tool to draw 2 curves, start one on the left and draw clockwise to the right.
now start a new curve on the left end of the first one, and draw anticlockwise to the right.
set that as a centerline cut and you will see it cut one part of it as a continuous line, and the other as 'backtacks', doing each line separately.

when generating gcode we try to determine which direction to cut, but if the direction changes halfway along the cut, what can we do?
very difficult in code to determine what to do. for example, how do we determine the difference between a 'circle' drawn as 2 freehand curves in different directions (if we know this, we can split it two parts) and a complicated set of intersecting lines which may or may not have loops and dead ends.

so, what to do?
a) manually split your cut lines into segments that are all in the same order. just a tiny break in the line is enough.
to do this you have to first generate Gcode, then look at it to see where the backtacking is taking place and create splits to separate them out.

b) use the Bezier Curves tool to 'create polyline' before you set it as a centerline cut. this fixes the order until you edit it again.
http://sketchucation.com/forums/viewtopic.php?t=13563

here is an example. I drew the top line left to right, and the bottom line left to right using the freehand tool, both start at the same point.
then set as centerline cut.
the bottom cuts 'in order' as one cut, the top does each segment then goes back to the previous one and does it and so on, what I call 'backtacking'.


to fix it, just split the lines at the points where the backtacking starts and ends, in this case 2 splits are needed.


I created a split on each end, about 0.5mm wide and now the cuts are good. 0.002" is enough of a split.
I have included the sketchup file so you can see what I did.

 

now, there is another way to do the pocketing for those shapes that avoids centerline cuts entirely
first, use the pocket tool to insert just the outlines, hold down CTRL key and click. that gives you a good continuous outline cut.
now insert some split lines to make that complicated shape simpler so that adding zigzags works.
if your stepover% is small ( less than 50%), remove the split lines.
if your stepover% is large then set the split lines as 'fold cuts' of the same % depth as the pocket.

Here is a demonstration

 

ah Yoram, you are right! knew I should have run the code through a simulator!

The retract prevent code only works for inside and outside cuts which are known to be loops.

In the case of centerline cuts, the beginning and end of the cut sequence can be anywhere, we don't know where, so we have to retract to avoid cutting into material that is not supposed to be cut.
In this case the cut is a loop but in the case of the zigzag in a pocket, it is not, and the code does not know that so has to retract.
Many centerline cuts are sequences of intersecting cuts where there are many 'end points' and no loop at all.

I don't see a way to improve it except to rapid down to 'previous pass depth' in a similar way to what happens after a tab. However, doing this is very complicated and prone to errors.

 

well, after a few hours of experimenting I think I found a solution, or part of the solution at least.
@mturneruk , install the attached, restart sketchup, then regenrate that gcode and see if it works.....