A time of 15/10 means that your opponent gains 10 seconds with every move they make. With only 6 moves including pre-moves the player would gain an extra minute. If their clock jumps up and down randomly it is due to lag.
http://support.chess.com/Knowledgebase/Article/View/121/0/what-is-lag-and-what-are-the-colored-lag-indicators-in-live-chess
Hello, I recently, actually just had a game with a culprit. A time adding culprit. Yes as I was playing the game with this person, his time was down to 1:22 and then jumped back up to 2:08. Keep in mind we are playing a 15/10 game. This happened about three more times with less than a minute left. Then things really got crazy. MY time went from 6 min....to 25secs! I actually scrambled to keep up but my time just suddenly was killed! He won the game from adding and stealing time. Maybe increasing his ratio and depleting mine now that I think about it. How can I report this person please this is absolutely frustrating.