From your original question there isn't enough strong evidence this is an hardwrae or software problem.
Hardware problem?
To eliminate most of the hardware origins, please run: Apple Hardware Test before attempting any other tinkering with the system.
Run it in extended mode and for at least 4 consecutive passes.
Once this 1st cause of trouble eliminated, please read the answer from Ashley and run Disk Utility from another disk than your system one. Most probably your internal filesystem is in bad state since you regularly crash your system. Hence at every new boot thsthe journal (of the filesystem) has to be used again to rebuild everything, and long processes killed have to start again their tasks.
Once this 2nd major cause of trouble eliminated, you will conclude that you most probably have a software problem, and it might be time to locate it.
Software problem
For this you will have to create a test account and use it for a serious amout of time, and with all the applications. Do you experience the same hangs with this test account?
If yes, then you have a general MacOS X misbehaviour. In this case, you have to reinstall MacOX X Lion.
If not, you will have to locate which application within your environment is causing this misbehaviour.
Activity Monitor will help you to identify what'swhat is going on when MacOS X decides to showstop being responsive. Until you will find the BBFHculprit, you will have to always run with this kernel spy on screen. Another
Another tool might be faster to identify the culprit processus. Open a Terminal or xterm window and type the following command:
top -o cpu
the top lines continuously refreshed by this command will show you if something abnormal is happening at the level of virtual memory or CPU usage, within seconds of the original problem.