Action Point Mistakes

In the game I am currently DMing, I decided to use Action Points. In previous games I had tried them and used the variants found in Unearthed Arcana. I found them to be massively overpowered and wanted to change them to something that PCs could use to keep from dying. So I made the following ways to spend them:

1) A single point adds a d6 to a roll.
2) Two points can be used to recover a spent ability like a spell slot.
3) Three points can be used to negate a critical. A critical instead becomes a normal hit.
4) Four points makes a save.

The point of these changes was to increase the survivability of the players, but in retrospect I feel they were a mistake. The ability to spend 2 points to recover an ability proved to be a problem because not every daily usage class ability is even. Extremely powerful use per day powers could be whipped out too regularly. Once the cleric gained Heal he was essentially unkillable as long as he had Action Points. He could make any save that would take his ability to cast a spell and he could cast a Heal if he took a lot of damage. The only way for him to be beaten was to really kill him in a single round.

Also the characters essentially became immune to save or die abilities. It was my intention that these kind of powers not decimate the characters with a few bad rolls, but these horrible abilities essentially became a non threat. Spell casters became much less of a threat than melee types.

So in retrospect I would not use this system again. I had the following idea that I think I may try next time I run a game. You can add 1d6 to a roll by spending 1 point. After that if you wish you can add another 1d6 by spending 2 points. And then another with 4, 8, 16, and so on. Of course, I would probably use this in conjunction with changes to death effects that I mentioned in a previous post.