Hi, im' using a livegrid and extjs 3.2.1

When i load the store "grid.getStore().load()", the mask is show, then the request is send, response is received, store and view are loaded, mask is hide... and1 second later, all the header disappears.
If the move is over a header, the label is show !

The probelm is only on ie7 (not in firefox) and it doesn't happen on each loading !

Please help