MacBook Pro not waking form sleep (sometimes)

My Macbook Pro is not waking from sleep (sometimes). To be very specific; the only time this happens is when it goes to sleep automatically, with the lid open, while plugged into the magsafe power cord. When I hit the space bar to wake it, the keyboard lights come on but the screen remains black. I do not have any problems with waking from sleep if I close the lid to force sleep (while plugged in). I also do not have any problems at all, if the computer goes to sleep while running on battery power. (I have had this MBP for about 18 months, the first year I had no issues at all. The problem started shortly after Lion was installed. (I also have a iMac with all the same software, almost identical set up, purchased at the same time as my MBP but have never had any sleep problems with the iMac, none even since Lion install.)

We do not have an Apple Store near us, but I have had the MBP in for service 3 seperate time to our local authorized Apple service/dealer. It is tricky to diagnose, because the problem does not happen every time. On average it won't wake from sleep 1 out of 3 times. Each time the service techs were able to re-create the problem but could not come up with a cause or solution, their only advice was to re-install Lion.

I have tried 2 different re-installs of Lion. (Both times erasing the hard drive first) The first time I did a clean re-install of Lion, then restored all of my data from Time Capsule. That did not work, the problem presisted. So I tried a second clean re-install of Lion and this time I did not restore from Time Capusle, I only re-installed critical docs from a flash drive and re-installed all other software (such as MS Office, Photoshop, etc.) from their original discs. I though this had fixed the problem but within a few days the problem re-occoured.

Any ideas would be appriciated, I have tried almost everything, I have days and days worth of time invested in trying to fix this issue. Thank you!