MacBook (late 2008) freezing, beeps on restart

Recently my late 2008 unibody MacBook (the version before the Pro monicker was put on all the laptops) has been having an issue where the screen flickers and freezes. Usually this happens when I am playing a movie on Netflix instant and the sound will go in an infinite loop as well, however the problem sometimes happens when I'm just surfing the web or if I move the laptop. Sometimes when this happens the laptop will reboot itself, but otherwise I have to do a hard reset. And sometimes upon rebooting the laptop will not boot up but will beep. I know this is supposed to signify a memory problem of some kind. I did replace the memory in the laptop several months ago and it was running very smoothly until recently. I have also tried opening the laptop and reseating the memory several times. When I do this, the laptop seems to be working fine for a while but then the bad behavior starts up again. I have also run Memtest86 over several iterations and no errors have been found. Also worth noting, this behavior started happening a few weeks after I upgraded to Lion. I have since then tried doing a clean install of Lion, but the problem has not gone away. Does anyone have any idea what the problem might be?

How many beeps?
Do you still have the original memory and have you tried it again just to rule out the new memory ?

Three quick beeps followed by three slow beeps. I still have the old memory but haven't tried putting that back in yet. I will give that a try and see if that resolves the issue.

Have a 13" mid 2009 MBP with a similar problem. Did reinstalling the old memory solve the problem?

Reinstalling the old memory modules fixed the problem. I have contacted the manufacturer of the new RAM modules to try to get a replacement.

Thanks for your quick response! Before reinstalling the old memory, did your MacBook boot-up normally after it was allowed to cool down? After my MBP has cooled down, it will boot normally. If the CPU temp get up to 72C, it will sometimes freeze and/or a distorted screen. When attempting an immediate reboot, it beeps 3 times.