Macbook shuts down instead of going to sleep

Since installing Lion, my 2008 white Macbook does not go to sleep either automatically after the preset time (15 minutes) or when I close the lid. Instead it completely shuts down and has to be restarted with the power button. This happens regardless of whether on battery or plugged in to power.

It seems to take much longer to boot up -- about 2 min. 45 sec.-- than it did with Snow Leopard. Login seems to lag several seconds after entering password. Any ideas?

(edit to add: I recently replaced the battery and upgraded the RAM to 4GB.