Timestamp skipping while direct logging

Hello,

I'm logging data coming on from VISA continuously. I take the data directly, convert it to string together with the measurement time stamp and write it directly into a log. The time stamps seems to skip ~6 minutes about every 6700 measurements. I do not do any timestamp arithmetic, I just take the value, convert it to string and write it to the end of a log file. Any ideas why this could be happening?