Image doesn't display until hover over

Weird problem in IE8. I have the following CSS:

.login-button .home-page-agent-link a:link, .login-button .home-page-agent-link a:visited {
color: #ffffff;
text-decoration: none;
font-weight: 700;
font-size: 100%;
display: block;
text-align: center;
padding-top: 15px;/*padding-top: 22px;*/
background: none repeat scroll 0 0 #042550;
float: right !important;
-webkit-border-radius: 15px;
-moz-border-radius:15px;
border-radius: 15px;
width: 180px;
height: 38px;/*height: 45px;*/
margin-top: 7px; /* fix button alignment */
}

The CSS selectors have been added to the CSS3PIE admin page. I have to check "Use PHP wrapper" to get it to work and also have checked "Display namespace as comment in css file".

When the page loads, I can see the image display briefly with the "non-css3pie" square corners. The images then disappear. However if I move my cursor to where they should be, they then display with the desired rounded corners. I can move my cursor away from the image and it continues to display correctly.

Obviously in IE9 the image displays perfectly so I can't see how this is anything but an issue with CSS3PIE somehow.