Mapnik Not Rendering Correctly

I'm having trouble with the mapnik rendering at high zoom levels.

As can be seen from the following images, the rendering is perfectly fine:

http://www.xlhi.com/z1.png
http://www.xlhi.com/z2.png
http://www.xlhi.com/z3.png
http://www.xlhi.com/z4.png
http://www.xlhi.com/z5.png
http://www.xlhi.com/z6.png
http://www.xlhi.com/z7.png

It's when you zoom right in, the rendering goes all funny as shown in these two screenshots:
http://www.xlhi.com/z8.png
http://www.xlhi.com/z9.png

As can be seen, the roads are no longer drawn, but the road names and parklands, etc. are shown.

I haven't a clue what's causing this! Is my machine too slow? (It's a bog standard AMD 1.5GHz with 512MB RAM).

Any ideas much appreciated.

Thanks in advance,

Eamorr