MacBook Pro :: Hard Drive Slow Down Than Spin Back Up?

Last night, after a few hours of using it (from 100% to about 8%), I tried turning off the computer for the night. After the dock and everything else disappeared, I got a spinning beach ball of death. The computer wouldn't turn off for at least 15 minutes. I had to do press the power button for 5 seconds to get it to turn off. I restarted it this morning and it seems to be working fine for now, but I'm scared of turning it off again.

Here's what I know: while the beach ball was happening, my wallpaper was still on the screen, the trackpad worked, but the brightness and volumes didn't, so I'm guessing I was logged out and no files were corrupted.

During the beach ball, the hard drive would slow down, then spin back up. I waited until it had slowed down a bit to do the hard reboot. I have to say that I reset the PRAM yesterday morning, thinking it might be good because I changed the hard drive to the Momentus XT last week and I read that you should reset it when you change hardware inside the machine. I did that before the crash, so when it crashed, it was the first time I tried to turn the machine off since resetting the PRAM, could that matter?

This morning, I went to disk utility and repaired permissions, there were probably a few hundred bad permissions, but Mac OS fixed them. I also tried verification, but it was alright. I then ran the daily, weekly and monthly tasks and everything went great.

What should I do? I already have a charger problem and a �screen is too blue whichever way I set it� problem. Do I have a lemon here?