I recently purchased a Kitchen Aid Professional 5qt mixer. I have burned out two 5 qt Artisans, so I thought a stronger motor would be better long term. Problem is that my buttercream is is no longer smooth, but very airy. I'm using Sharon's recipe exactly as before but it isn't working in this mixer. Any suggestions?

I've made recipe in both KA Artisan tilt head and KA Professional 6 quart bowl lift.

When you make the recipe in your new KA:  

is the bowl exactly the same as the bowl for your old mixer or is it a different shape? 

does the recipe hit the same places on the sides/beater as it did on the old KA?

This 

Let's try that again. I'm on my phone so it's taking longer to load I guess. This bowl is wider and not as deep as my artisan bowl. It gets very full but it does have gaps on the sides of the bowl. 

That's probably why your frosting is different.   See if you can use your original bowl on the new unit.

Unfortunately I can't. It's the lift kind and my old one was the tilt. :/

make a bigger batch so the air can't mix in -- get your bowl as full as it was before in the old bowl -- get the mixture as high up on the paddle as it was before kwim -- y'know maybe freeze what you don't/can't use right then -- you'll be ahead of the game

The bowls are not interchangeable between the two models of mixers.  With the stronger motor of the new mixer everything I have read says to accommodate the extra power by mixing at a reduced speed, thereby reducing the amount of air in the final product.

Well, we've managed to rule out the easy possible solutions.  As K8memphis say above, since the key to "no bubbles" is to get the mixture as high up on the paddle as it was on the old mixer, that's your last resort to try and get the identical results.

Since both old and new mixers have 5 quart capacity bowls, it may require a  different recipe quantity for the new mixer.   For my recipe of her frosting, I have notes saying "single batch", "double batch" and "triple batch".  Try making a triple batch of the recipe instead of a double batch and see if the new bowl capacity can handle that quantity of ingredients.  (Keep a towel handy in case of overflow!)

I was astonished at the difference in capacity when I purchased a second KA with a 6 qt. capacity.  I now make triple batches of recipes instead of double batches and love it!   I freeze any excess. I  My first KA is a red KA Artisan tilt-head, 5 qt.   My second darling KA is a bowl-lift, 600 Pro, 6 qt.      I rarely use my original tilt-head mixer anymore.  (I'm sure she feels neglected, poor thing....)