Wrong collecting data for unique visitors per month - need help

I manage a websites with a few millions monthly visitors and to collect data using the Piwik (1.8.4) and everything worked well. However, recently appear a problem with which I need an assistance.
There were problems in obtaining the number visitors per period - month.
A concrete example of one of the sites -
usually exceeds the number 600,000 UV, but now a few months of Linear increases only the first week to about 200,000 UV and then load each day UV resets and starts counting from the number which UV reached the end of the first week. For recieve data I use this
Has someone ever met with this problem, or anyone has idea what it could be caused by? I will be happy for your any comments

Can you please show screenshots that will demonstrate the problem? thanks

I’m not sure if it can be represented graphically. What exactly you mean? Only one what I have in my mind is some screenshot of monthly statistics one of website.
After 2012.06 is possible to see a significant decrease. There is a number which reaches the UV during the first week

Sorry I dont understand the problem, to be able to help you, we need more info… There is no known bug regarding this. You can also try: 301 Moved Permanently