I think it might be fixed if:-
1) it were not possible to end your turn in stealth without using a power such as walk in shadows or an invisibility potion, or AT LEAST using tactical retreat to a totally dark area. Those all use up APs.
2) Maybe you should retain your backstab bonus on the same foe for a single round (ie. you can't re-stealth but you can take a single guy down probably all the way).