front end vibration

I have a 2000 lincoln continental, 80,000 mi, i've replaced the control arms, had the front and rear ends aligned twice this month, and replaced and balanced tires...twice this month, but still i get a severe vibration above 65 mph...seems smooth till then. It seems to get smoother above 80 mph. The ball joints are still within limits...or so I'm told and the front struts are still good. I have spent 6 trips to mechanics (ford dealership and sears) to get this fixed...but still can't figure it out. HELP?!

I travel in Michigan often, and the speed limit is 70...so when i go slower, i actually impede traffic. Also, the vibration is pretty sever...the whol;e dash is shaking...afraid i'm going to vibrate other things loose. Thanks again for any help....everyone else seems not to know, and I don't want to replace ball joints HOPING that may be the problem...especially if they are still right in the middle of allowable specs.

thanks again