Why is my macbook pro experiencing constant hangs and crashes

Have you tried repairing your hard drive and permissions?

If that doesn't work, I'd reinstall 10.8.3 over your current install.

If that doesn't work, I'd back up my documents, erase the drive, and do a clean install.

Does it kernel panic, or just freeze and crash applications?

If they say it ran without problems, what hardware do you connect to it that you didn't have connected when they checked it?

How do you connect to the internet? Do you use some sort of USB aircard or other dongle?

If it is while using a browser, any browser, then it could be one of the other plug-ins in the /Library/Internet Plug-ins folder.

Try dragging them all to a folder on your desktop and start Safari. If all goes well, add the plug-ins one by one and see what happens.

If it doesn't help, you can move them all back into the folder.

Some people who updated from 10.8.2 have had problems that were resolved by installing the 10.8.3 combo updater from the Apple Support download page. I'd suggest a complete reinstallation in Recovery mode. Back up all data first.

That file couldn't possibly affect anything but Chrome.

If the re-installation used the original user account, the problem may just have been brought back. Perhaps resetting Safari and clearing the cookies may help, also, disabling add-ons and updating Flash is a must, if you use it. I would use the automation script in OnyX to clean out caches etc, but some people are averse to using third party tools like this.

As putnik points out: Did you do a clean install, or transfer from a time machine or other backup?

If the system is crashing in safe mode after a reinstallation, then you have a hardware fault. Take the machine to an Apple Store for testing.

There are two reasons why a hardware fault might only be manifested after an update:

1. The fault existed before, but a change in a driver or some other system component (maybe a new bug) brings it out.

2. A hard drive that was on the verge of failing is pushed over the edge by a sudden burst of write activity during the update. This is the most common scenario.

If this is getting to look like a disk failure, it is important to backup your system onto an external disk as a matter of priority. I suggest using Carbon Copy Cloner (CCC). If the cloning process throws up any error messages about bad sectors, it is a sure sign the computer disk is on the way out.

You should download a fresh installer and copy it to the clone, as you can do more thorough testing from outside the system. But your own data and media is the most important thing..