Gauge image missing seemingly randomly


 I am using a number of gauges in a dashboard set up (asp.net 2.0, IGv8.1) and occasionally a gauge will not show up when the page loads.  If I refresh the page the gauge then shows.   My deployment scenario is as follows:

        UltraGauge1.DeploymentScenario.ImageType = GaugeImageType.Png;
        UltraGauge1.DeploymentScenario.DeleteOldImages = true;
        UltraGauge1.DeploymentScenario.Mode = ImageDeploymentMode.FileSystem;
        UltraGauge1.DeploymentScenario.ImageURL = "~/images/SalesForce/Gauges/TargetKpi/Gauge_#CLIENT_#SESSION.#EXT";
        UltraGauge1.DeploymentScenario.FilePath = "~/images/SalesForce/Gauges/TargetKpi";

 The image url on the initial page load is:


And then when I refresh the page it is:


You can see how the timestamp (I'm guessing that's what "t" is) has changed slightly.  This behaviour happens both locally, and when my site is deployed on a server.  I'd say about 95% of the time the gauge image appears correctly, and the other 5% I need to refresh the page.  I don't think an image has ever failed to load for me on refreshing.

Any help on this would be greatly appreciated.