Blinking button on WPF application

My WPF application has a style manager that I have built on blend.

My problem is this: I've got a login button that blinks occasionally and i can't figure out how to remove this behavior.

Here's the style code for my login box:

<Style x:Key="LoginBoxGrid" TargetType="{x:Type Grid}">
    <Setter Property="Background">
        <Setter.Value>
            <ImageBrush ImageSource="/Client;component/Assets/images/LoginBox.png" Stretch="None" TileMode="Tile"/>
        </Setter.Value>
    </Setter>

    <Setter Property="Opacity" Value="0.765"/>
    <Setter Property="Width" Value="411"/>
    <Setter Property="HorizontalAlignment" Value="Left"/>
    <Setter Property="Margin" Value="126,150,0,111"/>
</Style>
<Style x:Key="LoginBoxHeader" TargetType="{x:Type Label}">
    <Setter Property="Grid.Column" Value="2"/>
    <Setter Property="Margin" Value="-16.183,18.347,0,0"/>
    <Setter Property="Width" Value="64.994"/>
    <Setter Property="FontSize" Value="16"/>
    <Setter Property="FontWeight" Value="Bold"/>
    <Setter Property="FontStyle" Value="Italic"/>
    <Setter Property="Foreground" Value="White"/>
    <Setter Property="VerticalAlignment" Value="Top"/>
    <Setter Property="HorizontalAlignment" Value="Left"/>
    <Setter Property="FontFamily" Value="/Client;component/Assets/Fonts/#Arial Black"/>
</Style>

<Style x:Key="LoginBtn" TargetType="{x:Type Button}">
    <Setter Property="Grid.Column" Value="2"/>
    <Setter Property="Margin" Value="16.6,9.022,9.282,8"/>
    <Setter Property="Grid.Row" Value="4"/>
    <Setter Property="Width" Value="164"/>
    <Setter Property="BorderThickness" Value="0"/>
    <Setter Property="Opacity" Value="0.78"/>
    <Setter Property="IsDefault" Value="True"/>
    <Setter Property="Foreground" Value="White"/>
    <Setter Property="Background">
        <Setter.Value>
            <ImageBrush ImageSource="/Client;component/Assets/images/LoginBtn.png"/>
        </Setter.Value>
    </Setter>
</Style>

And here's my code for the window:

<Grid Style="{StaticResource LoginBoxGrid}" >
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="0.127*"/>
            <ColumnDefinition Width="0.326*"/>
            <ColumnDefinition Width="0.462*"/>
            <ColumnDefinition Width="0.085*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="0.269*"/>
            <RowDefinition Height="0.028*"/>
            <RowDefinition Height="0.256*"/>
            <RowDefinition Height="0.223*"/>
            <RowDefinition Height="0.178*"/>
            <RowDefinition Height="0.046*"/>
        </Grid.RowDefinitions>
        <Label Content="User Name" Grid.Column="1" Margin="43.986,23.1,8,8" Grid.Row="2" Width="82" BorderThickness="0" FontFamily="Arial" FontWeight="Bold" FontStyle="Italic"/>
        <Label Content="Password" Grid.Column="1" Margin="43.986,15.873,8,8" Grid.Row="3" Width="82" BorderThickness="0" FontFamily="Arial" FontWeight="Bold" FontStyle="Italic"/>
        <PasswordBox Grid.Column="2" Name="PassTb" HorizontalAlignment="Left" Margin="8,18.877,0,8" Grid.Row="3"  Width="172.6" d:LayoutOverrides="Height"/>
        <TextBox Grid.Column="2" Name="UserTb" HorizontalAlignment="Left" Margin="9.282,23.1,0,11.004" Grid.Row="2" TextWrapping="Wrap" Text="TextBox" Width="172.6" d:LayoutOverrides="Height"/>
        <Label Style="{StaticResource LoginBoxHeader}" Content="Login" />
        <Button Name="LoginBtn"  Style="{StaticResource LoginBtn}"  Content="Login" />
    </Grid>

The Button I'm talking about is called "LoginBtn", as is its style.

How can I remove that blinking behavior? Thanks in advance.