Welcome to my world pepper. My mouse and computer have an off and on relationship. So I scroll with 2 hands, one on the mouse and the second it freezes I give it a push with the touchpad. Only thing that will free it up.
This HP Laptop is a complete lemon. If the screen starts to flicker I have seconds to plug it in before everything completely dies. One of the features of this lemon was a quick start up. Hah! I don't remember when it snuck out the window but I can fetch a cup of coffee, put in the milk, zap in the micro to bring it up to par and walk back to the desk and it is just arriving at the start screen.
All I can suggest is that you are diligent after making the word bold, be sure the 'B' icon is not highlighted when you go on to the next word. I will often highlight a word, look up to the icons and in that split second the rest of the sentence picked up the motion of my hand over the touchpad and POOF - everything changes.