The following should give you some ideas:
While it is for 10.6 the same techiques apply equally to Lion except that where it talks about installer dvds you have the lion recovery parition.
See if you machine crashes when you boot in safe mode.
If you're able to boot, launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ If you’re running Mac OS X 10.7 or later, open LaunchPad. Click Utilities, then Console in the page that opens.
Select the most recent panic log under System Diagnostic Reports. Post the contents — the text, please, not a screenshot. For privacy’s sake, I suggest you edit out the “Anonymous UUID,” a long string of letters, numbers, and dashes in the header and body of the report, if it’s present (it may not be.) Please don't post "shutdownStall" or "hang" reports.
If you can't boot in the usual way, try a safe boot. The instructions provided by Apple are as follows:
- Be sure your Mac is shut down.
- Press the power button.
- Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
- Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
During startup, you’ll see a progress bar, and then the login screen, which appears even if you normally log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Safe mode is slower than normal, and some things won’t work at all.
Boot from your recovery partition by holding down the key combination command-R at the startup chime. Release the keys when you see a gray screen with a spinning dial. When the Recovery desktop appears, reinstall the Mac OS.