KitchenAid Mixer head wobbles

I have the KitchenAid Mixer Model KSM180QHSD which is just a 100 year edition, tilt head 5 quart mixer. I bought it new and it's about 3 years old and is only used about once per month. The problem is that when I use it, the head wobbles even though I have it in locked position. I've done the dime test to ensure the beater is not hitting the bottom. I've also checked that the pin is not sticking out. This issue is most noticed when I'm creaming cold cubed butter and sugar, or when the dough consistency gets thicker. Any ideas what the issue is?