Aqua display problem

Hi there

I've got a problem with Aqua not displaying properly. Symptoms include:

-the menubar is pure white, rather than the aqua effect (either blue or graphite)
-the horizontal ribbed background pattern on windows and dialogue boxes is invisible
-Light colours are invisible, or almost invisible
-Text is poor quality, with coloured haze
-highlighting is barely visible.
Basically, everything is very washed out. Note that this is definitely a software problem because occasionally, when launching an application or switching user, the menubar displays correctly, and then reverts to the whited out version. Restarting doesn't solve the problem.

I've recently acquired a 19 inch flat screen monitor, but the problem remains on the Powerbook screen as well.

I'd had a similar problem before with one login, which I deleted and re-created, solving the problem. This time it's all users. Perhaps the problem is linked to some bad preferences file associated with login/start up? Can anyone suggest any preference file I might try trashing?

Any other ideas as to the possible cause?

Thanks!

Duncan