Shift key went berzerk, scroll wheel on external mouse stopped working

Dear iFixit,

Two days ago the scroll wheel on my mouse stopped working. Hours later, it was as if one of my SHIFT keys was permanently stuck down. EVERYTHING I TYPED WAS LIKE THIS. When I restarted the computer (13" Macbook Pro Mid-2009 2.26GHZ) it would act as if the SHIFT key was on, and start in safe-mode. I brought an external keyboard from work, which would type normally once logged in - however the keyboard isn't recognized until safe-mode is already loading. I used the external keyboard to log in with my password, otherwise it was impossible to type due to the stuck SHIFT. Once logged in, I could type normally on the external keyboard but if I moved to the built-in keyboard everything WAS IN CAPITAL LETTERS. :<

I wanted to reset the PRAM and SMC, but I couldn't do so because the computer would always recognize the shift key as being down - sabotaging the key combinations that would properly reset PRAM / SMC.

Of course, I also looked under the keys physically (took of the SHIFT keys and caps lock key). There was nothing in the way, nothing stopping the key from being pressed or pressing the key for me.

HOWEVER, here's the good part, I fixed it. While logged in safe-mode, with external keyboard / mouse attached, I opened the "Keyboard Viewer" from the Language & Text menu, with the thought that I might be able to see whether one of the keys was registering as always pressed, or unpressable. (Keyboard Viewer displays your keystrokes). As I pressed SHIFT (both) and caps-lock alternatively, the keyboard viewer would show me that they weren't being pressed, or at times were being pressed, with capitals being displayed randomly. I pressed space-bar and the viewer showed the right hand shift button being pressed at the same time, although of course it wasn't. However, despite all these random results... while playing around, I found that it had re-organized itself into normality. From the keyboard viewer, both shift keys were working properly and caps lock as well, with no extra problems. I exited the keyboard viewer, typed a few test messages in Textedit, and everything works fine. MY SCROLL WHEEL EVEN WORKS AGAIN. :O

And so, I am typing this message on the in-built keyboard and scrolling around happily. I have also reset PRAM / SMC just for kicks.

I do have a QUESTION, since I can imagine that this will happen again. Based on the description, what do you suspect is going on? How come my laptop 'think' and get confused about key presses... remember that shift is on from the moment it boots, etc? What would cause such internal non-mechanical mix-ups? If logic board or something like this, how could testing the keyboard viewer restore the keys?...

Your thoughts are welcome.

Best,

Martin