I had some other comments that I forgot to post before.
Overall, it's quite an interesting system. However, I can see several issues with it:
1. There are many stunts that are immediate interrupts or are done as part of another action (like Knock Em Down, Leaping Charge, or Timed Attack), so there is no reason not to use them when you get the chance. This essentially turns it into "if you have this skill, you have a chance of getting an attack bonus in such-and-such a situation" and could remove a lot of the "flavor" you are going for.
2. You can use these a LOT in an encounter. For example, it's quite reasonable for a character in the middle of the paragon tier to have almost a +20 (or more) to a skill (+7 level, +6 stat, +5 trained, and possibly racial and item bonuses). This would mean that the character would have a good chance on most of these even with a -10, so he could use them three times per encounter. And this is separate for every skill, so if he had five trained skills then he could use fifteen stunts per encounter! And that's not all because he has a big enough bonus from level alone to have a good chance of pulling of stunts from skills he isn't even trained in.
3. Relating to the above, the system requires keeping track of a separate count for every skill, which could easily get cumbersome.
4. Many of the effects are relatively low, such as a +1 or +2 to one die roll. Making one roll just to get a small bonus to another roll adds complexity for relatively little effects.
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What I would recommend is to have one encounter power that all characters get called "Use Encounter Skill Stunt", and one daily power that is called "Use Daily Skill Stunt." Both of these would operate like Channel Divinity in that there are many different effects the power can have but you choose one of them each time you use it, and you can only use it once per encounter (or day). Optionally, you could provide more uses at higher tiers. Then you can make the stunts more powerful to compensate for the reduced rate at which you can use them. When designing these stunts, it might be a good idea to come up with a standard for comparison to figure out the DCs - let's say, a stunt as powerful as an Nth level utility power has a DC of 20+N.
It's probably also a good idea to make all skill stunts Trained Only.