= true, reloads tiles EVERY time

I have a globe with several imagery layers (WMS) on it. As I cycle through them (keyboard arrow keys), I can toggle the show parameter just fine to show and hide them. But once a layer has loaded, if I toggle show=false and then show=true, the globe re-loads all the tiles (i.e. makes a new web call) even though the globe view hasn't changed. Is there anyway to prevent this? I don't want to have to re-download all the layer tiles each time I hide and show a layer.