I'm just taking a wild guess here, there's so many different things that could be wrong.  It almost sounds like you have some corrupted files, maybe caused from dirt or scratches on your Sims2 disk or possibly the disk reader itself may need cleaning.  Another possibility is there may be some remnants of the Pets EP lurking around that the uninstaller missed.

MTS2 has a tutorial for restoring neighborhoods here:

http://modthesims2.com/showthread.php?t=138975&highlight=afresh

SimPe can be found here:
http://sims.ambertation.de/

Jase gives a pretty good explanation of what's going on here:
http://www.insimenator.org/showthread.php?t=1314&highlight=pagefile
That should get you pointed in the right direction.
Here's a few things you can do to help improve game play:
Set shadows to OFF in graphics/display settings of the Sims2;
In game options, turn off neighbors and set view distance to small;
Defrag your hard drive regularly;
Shut down all programs running in the background that you don't need (instant messengers, etc.);
When you notice the game starting to slow down, reboot your system.   This will help free up system RAM and resources;
runs Sims2pack Clean installer and get rid of duplicate files in your Downloads file (they'll be highlighted in pink);
Run the Sims 2 Hack Conflict Detection Utility from http://www.simwardrobe.com and resolve any conflicts it may find.  (Sometimes conflicting hacks will slow your game down noticeably).
Those are the best tips I can give you without knowing your system specs and what EP's you're running.  :happy7:

Check out this article on Gamespot about Sims2, graphic cards and performance.  http://www.gamespot.com/features/6133243/index.html?type=tech&page=1
Basically what they're saying is the bigger the CPU, the better the performance.  Adding RAM and having lots of free disk space helps alot.  Also defrag your hard drive often.   Hope that helps.

More than likely your monitor or vidcard went bad.  My monitor did the same thing a few weeks ago, turns out it's number was up.  Your vidcard may be overheating as well.  Never hurts to clean your cooling fans, any openings in the case that let air circulate, and the heat syncs with some Qtips.  Also check to make sure your vidcard is securely in it's slot on the motherboard.