Replying to Life Span of a camera: DLSR?

Hey,
I've been shooting with a Canon Rebel Xti for the past 3 1/2, almost 4 years now. Last time I checked its' shutter count was somewhere between 150,000 and 175,000. Been a good camera, I've treated it well and all that, but lately I've been having some issues. Firstly, from time to time auto-focus craps out on me and goes away for days on end only to show up again and work flawlessly until it stops working again. This kinda sucked, but it was something i could easily live without, no issues for me being forced to shoot manual focus sometimes. Now, the past couple weeks, another issue has come up, this one is a little harder to ignore. This first happened a couple weeks ago, but after firing the shutter an error screen comes up: "ERROR 99. PLEASE TURN CAMERA OFF AND ON AGAIN." This happened a few fleeting times and i kind of ignored it, hoping it would deal with itself. Wrong. Today shooting literally EVERY OTHER shot i went to take i got this unpleasant message. So I ask you, has the three and a half years of using my camera heavily taken its toll? Is this he way it works, do I need to start looking for an upgrade? Or is something just wrong with mine that could (hopefully) be repaired?
Anyways, thanks a lot for reading this, and I appreciate any answer you guys may have.
cheers!