Casting, Concentration DC's and interruption modifications

[TheDarkSaint]

I'm personally unhappy with how 3.5 deals with casting and trying to interrupt a spell. I started in 2nd, where it was simple to ruin the concentration of a caster. I like the idea of the concentration skill, which gives a caster more of chance to get their spell off, but it seems to me, as it stands, that it is just too simple to jack the concentration score out of the roof and not worry about interruptions.

I'm thinking about the following.

Spell Casting

When a spell caster casts, they must make a concentration check for the spell to go off. The DC of the check is the current spell level x5. If a caster takes any damage in a round before their spell fires off on their initiative, the DC of the spell casting is modified so that the amount of damage is added to the DC check. All damage is cumulative to the DC check through the round until the casters initiative.

The way I see it, it scales as the caster progresses, so a 5th level spell has a DC of 25, but a 9th level character should be able to make that kind of skill check with some sort of regularity in a skill that should be primary to their class, but it gives enough wiggle room to make them sweat. Any damage they take would really push the DC up, making it quite difficult to get higher level spells off with regularity.

Is this too harsh? I'd like some feedback before I put it into place with the new group.

[docnessuno]

It's, quite frankly, one of the worst houserules i have ever seen.

The only problem with 3.5 concentration is casting defensivly: the DC improves by 0.5/level with the skill improves by 1/level, so on higher level characters the check is trivial.

If you want to "fix" concentration, the pathfinder version works fairly well.

[Waker]

How about this.
Concentration DC = 10 + Spell Level + Total BAB of Opponents Threatening the Caster
Damage done to the caster while he is casting a spell automatically interrupts the spell. Damage done before or after have no effect on either the DC or the process of spell casting.

[Flickerdart]

The problem with that "fix" is that it becomes impossible to cast a spell after you've been hit by anything competent, meaning that casters will have to either win battles before a shot's been fired (and nobody else gets to have fun) or be useless forever (and then they don't have fun).

[Sliver]

I don't like the idea of a character not being able to use their class features even if they hit nat 20 without investing feats and money or a minimal ability score. Because at 9th level spells, you'll need to hit 45, which at level 17 you can only get to on nat 20 with +5 constitution. Sure, it's not that hard to get that con score, probably expected by that point, but that's 5% to use your class feature, when you aren't even interrupted! Yeah, a wizard will have higher, but how much higher without him feeling forced to take specific investments that he otherwise wouldn't?