Why does my cursor keeps jumping around in word

I purchased the new Office 2013 to work with the Windows 8 that came with my newly purchased computer (Lenovo.)

The issues is, which has been driving me crazy over the past two months, when typing on the Word 2013, my cursor keeps jumping around on the page. For example, in the middle of my typing a sentence, the cursor would jump to three lines above in the middle of a word and continues the my input there. Whenever I see my writing in word is not changing while I definitely was typing, I will have to stop and started to examine where the words have went. After I find them five or whatever lines above, I will have to delete them, use the mouse to click the end of the writing again and trying to retype and continue to type.

I can swallow the problem if it only happens once each document, but it happens 10+ times when I type a two-page essay. I don't think the newly developed software is making my life easier but the opposite.

Does this happen to anybody else? Any clues of why or any suggestions, please?

Thanks!

P.s., please note that besides the letter keys I hit on the keyboard, I did not hit on any other keys such as arrow keys. Especially knowing my cursor jumps around, I am extremely careful with hitting my keys. I do not touch the mouse pad, neither.

P.s., besides jumping to other places and start insert words, sometimes the cursor would select/highlight a line or half line. If I was not aware of the change and hit a space, the whole selected lines are gone. See, I didn't know how to select lines besides using the mouse, and I did not use the mouse but simply typing, so I am very frustrated with what my cursor does to me.