Inner sidewall of tires constantly cracking
 

I've had my A4 for just over a year now and have gone through 7 tires. All of them have happened the same way. The inner sidewall starts cracking little by little.
When I first purchased the vehicle, it had 20 inch aftermarket rims on it. After going through 6 tires, I figured the size of the rims were to big for the car and causing the flat tires (since I had to buy skinny tires, most likely the weight of the car was putting pressure on them and causing cracks).
So about 5 months ago, I got some stock size rims put on it, with brand new Pirelli tires. A few days ago I was driving on the highway, when the front driver's side tire blew, leaving nothing but shreds of tire around the rim.
From the way the tire came off, it seemed to have experienced the same kind of cracking.
I wonder if there is something else that is causing the cracking sidewalls. I have always noticed that at highway speeds the car shakes a bit, even after getting an alignment and balance/rotation done.
I did bring it into Firestone once, when I had the aftermarket rims on it, and they told me it had spacers on the front wheels, to make the tires fit. Could this have something to do with it?

Before I got all the way to the bottom, my first thought was the spacers, since you mentioned 20's. I don't know the factory offset/tire/wheel-size of your car, but it should be pretty easy to find with google or bing or any search engine, honestly. Find out what yours is, and buy the correct wheel/tire combo for your car. Have them remove them stoopit spacers, too, while they're mounting your new wheels.

If that doesn't solve the problem I'd say you might have vandals screwing with you.