Time Machine only keeps two days backup, displays old backups that don't exist. Occurs on two separate HDD's

Question

I have a 15" 2010 MBP C2D-2.53 in completely original form except it is running OSX 10.8. Applecare just ran out so that's not an option, FYI. Lately I went to fetch an old file from TM because the current file was corrupt and I noticed that TM was only backing up for the past couple of recent days. Oddly enough, it also displayed a couple of backups dated from 2010, even though the external HDD I'm using was purchased 2 months ago and the files displayed in the "2010" backup did not exist at that time so it's an impossibility. The backup disc is a toshiba canvio external 320gb USB.
I thought that was odd so I zeroed and reformatted that disc using disc utility. I made it the TM disc again, did the usual backup without error and waited a couple of days. Sure enough, when I checked it had continued the trend of displaying the impossible 2010-dated backup and was still only backing up the past couple of days' material correctly. I decided that this was all getting a bit silly, so I plugged an old external WD USB hard drive, formatted it and made it my second time machine backup disc. Waited a couple of days, accessed it, saw the same exact thing as with the other external drive.
I'm perplexed, however based on these rudimentary tests I'm guessing the culprit lies with the machine rather than the external hard drives. These facts may be unrelated, but I also notice that when I go to eject the toshiba external drive, it says something like "there is more than one partition, would you like to eject one or all" and the machine also had some work done on it by Apple recently. Specifically, a new logic board, airport card and complete erase/install where I reinstalled data successfully through migration assistant. The work was done due to kernel panics at boot. Thankfully for now I've also got CCC cloning the drive, but I do require the long-term snapshots that TM takes.
Any help would be greatly appreciated, and I'm willing to do some digging to fix this issue.

Answer 1

Please read this whole message before doing anything.
This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.
Step 1
The purpose of this step is to determine whether the problem is localized to your user account.
Enable guest logins* and log in as Guest. For instructions, launch the System Preferences application, select Help from the menu bar, and enter “Set up guest users” (without the quotes) in the search box. Don't use the Safari-only “Guest User” login created by “Find My Mac.”
While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.
Test while logged in as Guest. Same problem?
After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.
*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.
Step 2
The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode* and log in to the account with the problem. The instructions provided by Apple are as follows:
Shut down your computer, wait 30 seconds, and then hold down the shift key while pressing the power button.
When you see the gray Apple logo, release the shift key.
If you are prompted to log in, type your password, and then hold down the shift key again as you click Log in.
Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs.  The next normal boot may also be somewhat slow.
The login screen appears even if you usually 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.
*Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t boot in safe mode.
Test while in safe mode. Same problem?
After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of steps 1 and 2.