My MacBook Pro sometimes doesn't wake up from sleep mode

Dear users, my MacBook Pro (model 5,1 - 15" Late 2008) is behaving oddly since I upgraded to OSX Lion. Sometimes, after going into sleep mode, it doesn't wake up. When I hit a key (space or enter), I see the login screen, but it's not possible to type my keyword. The only thing active is "the spinning beach ball of death", so I have to manually shut down my Mac and start over. This problem (and also other problems) did not happen in Snow Leopard. The only thing that changed in my Mac is the OS; other programs remain the same. I already tried reseting PRAM and SMC, and fixing disk errors. Thanks for your attention.