Great idea!
I don't like the current way stun is handled, either. Simply too hard to resist for even experienced characters. Your house rule sounds pretty playable to me. Definitely worth the idea point.
This actually made me think a little about the whole thing:
Another approach might be to make it a cascading resistance roll with lets say 110/ +10 per round, so 5 rounds would result in:
RR: Stamina (160) - no stun
RR: Stamina (150) - 1 round
...
RR: Stamina (120) - 4 rounds
all other results - 5 rounds
A tiny bit more math to do, but everyone should be able to cope with it. This rule would allow for more differentiated results. Please, don't be to hard with your critique, it just sprang to my mind.
By the way, if you think "stun" is a bad choice in English, you've never heard it in German (Benommenheit). Its even worse.
I looked up stun and staggered in the dictionary. They're synonyms for someone being shocked and surprised, with stun having the connotation of "unconsciousness", while stagger has more to do with surprise. So stun might be a better choice, but hey, that's just phraseology.