SBBOD on Clean install

I've been trying to help my fiance fix her macbook pro. It is about 3 years old, and running Mountain Lion. Due to slow performance and consant spinning beach ball, I reformatted the harddrive and installed a fresh copy of the OS. We still get constant beach balls.

I have been tracking her activity monitor and when it hangs up the top memory using program says "kernel" something or other. It is also important to note that it happens randomly, no matter what program is open or closed. We have tried closing all internet browsing... but something as simple as word processing causes hang ups.

I have done all of the first aid options under disk utility with no success.

I have read that the problem could be hardware related. I can't see why memory would be too low, since it is simple programs causing the stalls. I am about to try and install a fresh copy of the OS on an external harddrive and see if that helps at all.

Please help with any ideas of what I can do... and please keep in mind Im a Mac noob, so minimize on the acronyms and speak like im a child, lol.