Solubility of Gold in low concentrations
Whenever you prepare very dilute working standards, it's good practice to do a stability study to see how long you can store them. There are many factors that can influence the stability of these solutions. The choice of container can make a big difference. Dilute metal solutions should be stored in polyethylene or polypropylene containers, not glass. Another well-known problem is photoreduction. Gold solutions should be stored in opaque containers and be exposed to light as little as possible. Some solutions may also be sensitive to atmospheric exposure. (though I don't think it's an issue for gold)

I don't have experience with working with gold in hydrofluoric acid, but such a large change in such a short period of time in the absence of the above factors may indicate some actual reactivity with your matrix, for which matrix-matching might help, though you may be better off reformulating your solutions if that's the case. It may also be an option to just make fresh working standards very frequently or increase storage life through refrigeration.