MacBook Pro doesn't wake if left asleep too long; sleeps immediately after boot; SMC Reset / NV Reset only temporary fix

I've tried hard to find someone else with these symptoms, and nobody seems to have a complete answer.

After upgrading my 2008 MacBook Pro to OSX Lion, these problems began.

Whenever I leave the lid closed for too long - not entirely sure how long, but the longer I leave it the more likely this is to happen - it refuses to wake when I open the lid. The power light will brighten, as if it were about to wake, and sometimes the screen will flash, but then it will all go black, power light and all.

At this point I press the power button, and the power light briefly flashes, and then the macbook seems to 'really' turn itself off.

Then I press the power button again, and it begins to boot. I enter my FileVault 2 password, and the apple logo and spinner appear, but as soon as the graphics driver kicks in (or whatever it is - the point when the shades of grey suddenly change and the brightness automatically adjusts) suddenly the mac goes straight to sleep immediately, and we're back to the same symptom as before.

It will do this repeatedly through a 2nd, 3rd, etc power cycle.

Resetting the SMC doesn't seem to help.

I have managed to get the macbook to boot normally by clearing the NV Ram, and also by booting into single user mode and immediately shutting down and rebooting. But this does not solve the problem, it merely lets me boot one more time.

Any thoughts appreciated, it's driving me mad, not to mention losing me work.