Mystery charset-set problem - sometimes OK sometimes not

[Jirka]

MySQL 5.0.11beta
Windows XP SP2
locale CZECH (cp1250)

I have "include file" on each page where DB is accessed, its like this:
<%
Set adoCon = Server.CreateObject("ADODB.Connection")
strCon = "driver={MySQL ODBC 3.51 Driver};server=localhost;uid=x;pwd=x;database=x"
adoCon.Open strCon
adoCon.execute("SET character_set_client=cp1250")
adoCon.execute("SET character_set_connection=cp1250")
adoCon.execute("SET character_set_results=cp1250")
%>

When I recieve SQL result via SELECT, it is sometimes in correct charset,
but after refreshing the same page
it turns good characters
ěščřžýáíé
into wrong
?š??žýáíé

when i keep refreshing the same page, it randomly show first or second case.

parametres in my.ini:
[client]
port=3306
default-character-set=cp1250

[mysqld]
default-character-set=cp1250

I am very unhappy about this, can somebody help?