Strong Grip Only When "Hitting Up" On Ball

Thats extremely impressive for an 18 cap (And a consistent 223 yard average straight with a 17* club would be impressive for a 10 cap IMO).  Are you sure this isn't just mental?  Meaning, sometimes we "like" a feel and think we hit it better that way when its really just the same.  Just throwing this out there - its very unlikely you are actually doing this with an 18 cap.

I do, but I am trying to change it.  I hit down with the driver and all the other clubs, and I hit a fade 95% of the time.  The key thing to understand here is that it isn't your swing that should change, its your setup.  If you want an upward AoA, move the ball forward.  Now, if your head is moving (more below) this doesn't work.  However, if you have a consistent swing bottom, you can get a postiive AoA by simply moving the ball forward (as it will be just in front of the bottom).

As usal, having a consistent swing bottom is the fix.  You get that by getting the first three keys IMO (steady head, flat left wrist, weight forward).

Its hard to tell without a video.  However, it is probably likely your head is moving forward in the downswing.  Taking a strong grip will close the face to the target, but leaves it open to the path, producing a fade.  A neutral grip probably leaves your face open/square to the target with an out-to-in path, producing a slice.

I had this exact same problem.  When your head comes forward, your angle of attack gets really steep, and you throw the club outside.  Further, when your head comes forward, you are shifting your swing bottom forward, which means you will never hit up.

What has worked for me to help this is to have a swing thought of keeping my chin glued to my chest and making half-swings.  Making full swings with this swing thought isn't great as you can get hurt, but little half swings can give you the impression of a stable head with some power.  This little drill has helped me a ton.

You should really post a video, though, as its hard to tell without one.  Your problem sounds like my problem (complete with straightening it out by gripping stronger).

Instead of just pounding away at 1200 balls per week, try putting an alignment stick in front of you about square.  Make half swings, concentrating on keeping your head still (but moving forward with everything else) and starting the ball to the left of the stick (for a fade).  This is a great drill to feel the proper clubface rotation with a neutral grip and the head staying still.  Hit 5 or 6 like this and then hit a full swing and repeat.  Isolating the problems (head moves forward, open face at impact) helped quite a bit.  It also helped me quite a lot to get an adjustable driver and put most of the weight in the heel.

That said, if you're hitting a 17* club 230 yards consistently striaght at a 18 cap I'd just strengthen the grip and go work on something else :).