Hi,
After lots of faffing around we've managed to come up with a configuration that works on the LPC4357 running at 204MHz using an SDRAM. I guess there are similar problems generally with the LPC18xx - if not then feel free to ignore this post ;-) but if so, you might find the info on this page useful:
<a href="http://www.lpcware.com/content/forum/working-lpc43xx-running-204mhz-sdram">http://www.lpcware.com/content/forum/working-lpc43xx-running-204mhz-sdram</a>
Cheers
Richard
Hitex UK Ltd