How do you correctly use a meat thermometer?
With thin cuts of meat (like chicken breast or a steak), you want to make sure you put the thermometer in to the thin side of the meat so that the whole thermometer goes in to the meat, rather than putting it in through the "top" so that only a little of the thermometer is in the meat. If you're putting it in the top, you can get wildly inaccurate temperatures. Also make sure that the thermometer isn't touching a pan, exposed to air, or touching a bone. Any of those can have a negative effect. 


Also, you only need to cook chicken to 74C / 165F. That may be part of the problem in your case. 

You may also want to check that your thermometer is accurate. You can stick it in boiling water to check that it reads 100C / 212F (assuming of course it goes up that high).