That's insane, how can you live with a transformer that does that? If that is normal behavior, it doesn't need repair, it needs replacement!
FWIW Vern, your test doesn't duplicate what he is seeing, that's more than a 5 amp load you're using, he sees a 3 volt change with a half amp change in current. You can sing all you want "that's how it works", but I say that's a crappy product if that is actually the performance!
My question is that actually the output voltage or just the meters being stupid? It's really hard for me to believe that all the people that use this transformer put up with that performance.