cursor jumps while typing text

First time on this forum.  I am a long-time PC user, not a new net user, nor an expert, but I can't find an answer anywhere.  I found this site so here goes.  I have a new Toshiba Satellite 1135-s155 notebook.  Since I got it, I have had a keyboard problem.  I will be typing text and suddenly the next character (usually alphabetic letter) or the next 2-3 characters I type, along with the cursor, jump to a previous word.  The letters and cursor may end up in the middle or the end of another word.  The letters might insert themselves somewhere on the same line, or on a line or two above.  I am being very sure that my fingers/hands are touching only the character keys I want to type.  This is happening even as I type the present text (I am correcting as I go along).  Here is an example:

 Sometf imes the first letter o

I was going to type "of".  The o typed properly, but the "f" jumped back into the word "sometimes".  It doesn't matter whether I am typing in MSWord, e-mail, Excel, or a format like this.  Can someone help?  This is making me nuts.  My livelihood depends on my ability to work fast, and this is really slowing me down.  Also, sometimes when I have gone  back to insert something in an earlier sentence and I am typing the text, the letter(s) will jump forward.  Additionally, sometimes text will inexplicably become highlighted.

Your contribution to fixing this and helping to restore my sanity is welcomed....Debra