This thread in Apple discussions. Late 08 unibody MacBooks -- including mine -- with a 4GB mem update hang and lock randomly, screen freezes, image breaks up and so on and only powering down (pressing the power button for about five seconds or so), and rebooting after awhile helps. So I'm wondering whether you have ran into this problem yet with your clients.

Personally, I'll probably have to contact the retailer and send my MacBook back, if this persists, which royally sucks.

I posted this in that thread:
I have had this same problem appear randomly and beginning under Leopard since July. I bought this late 08 Unibody in early June and went on for about at least a month without problems.

Then this started happening randomly, meaning it doesn't seem to matter, what I'm running at the moment, and now it happened this morning after maybe 15-20 minutes from boot, also happened yesterday and the day before that, so it appears to become more frequent.

The 4GB of RAM was installed by the retailer and it is manufactured by Kingston specifically for these machines. And FWIW, I have done complete updates, including firmware and boot ROM and this was happening already with Leopard, and the firmware and boot ROM updated. If it is not defective or incompatible RAM, then it is most likely the video, NVIDIA GeForce 9400M.
Yes. I can haz a biz and site, no?