my website has some messy code ,just in sometime some where

sorry for my poor English

my site is ASP.NET WEBFORM + SQL SERVER +windows server 2008 with Chinese

page Encoding is utf-8

some times the same line text,have some messy code,it is not all one page is messy code ,in a line text,just one or two character are messy code,please look at the picture, the "???" is the messy code,

the messy code picture 1 the messy code picture 2

and the messy code disappears in some hours later but I did nothing, and it will apperar in some hours later. please help.

do you understatnd ? my English is very poor ....sorry.