The rule stays the same except that in each rolling critical attempt after the first, the total attack bonus is reduced by 1. So if the attacker first rolls the 19-20, then the first critical confirmation is at his full attack bonus (let's say +10). The 2nd is at 1 less (+9); the third is at 2 less (+8), etc. It would stop decreasing when the bonus reaches +0, but if the rolls keep hitting at the +0 bonus, then the rolls continue. A roll of 20 would continue the rolling regardless.

This continues to allow for the possibility of an awesome attack but reduces the likelihood.