Upgrade to Lion and then to Mountain has caused me problems

Question

After upgrading to Lion a year ago or so I developed a problem on my MB Pro that I'd never had before. (1) My Mail app starting acting funny. I'd compose an email then click send, but it would sit in the Outbox--for hours, even days. I had to login to MobileMe to send mail and to get Mail. And (2),  my system would frequently crash. If I left 3-4 apps open and then went and had lunch, for example, when I came back, my system had crashed. The screen was blanked out, and I had to move the mouse, kind of use it like an eraser to erase a part of the crap in the foreground of the screen so that I could see my login, after which I would enter my password and then login. After login in I would find that all of my apps had either crased or were frozen. I thought it might be that I needed to upgrade my Memory, so I upgrade from 4 GB to 8 GB, but this didn't do any good.
So...after Mountain Lion came out, I upgraded, hoping by some miracle this might solve the problem. But no, the problem persisted. I had AppleCare and so I called Apple three times about this problem, but they couldn't solve it either, although one senion tech adviser did manage to get my Mail app working again. He went to some folder (I wish I'd noted what this folder was at the time but I didn't), found there was sever gobbly-gook or junk files inside it, and then proceeded to delete them. After this my Mail app worked OK but the other problem persisted.
A week ago I backed up everything on my MB Pro to my Time Machine BU, bought a new Mac Mini, and then used Migration Assistant to import all my data from the  Time Machine BU. I was hoping for a miracle--i.e., hoping that this problem would disappear. But not only has the problem not disappeared, I now can't even use my Mail app on the new Mac Mini. It is completely disabled...plus I still get the random crashes. I am getting by using my old MB Pro. I've transfered back to my MB Pro my essential files and I am using it while I try to figure out what in the **** is going on with the original data on my Mac Mini.
Any ideas would be much appreciated.

Answer 1

Install or Reinstall Lion/Mountain Lion from Scratch
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Erase the hard drive:
  1. Select Disk Utility from the main menu and click on the Continue button.
  2. After DU loads select your startup volume (usually Macintosh HD) from the left side list. Click on
      the Erase tab in the DU main window.
  3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on the Security button and set the Zero
        Data option to one-pass. Click on the Erase button and wait until the process has completed.
  4. Quit DU and return to the main menu.
Reinstall Lion: Select Reinstall Lion/Mountain Lion and click on the Install button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.