Wow! They didn't last very long huh!
My stock 90x macadams started that damn head shake at around 3-4K. I went with Bridgestone Battlax 020's.... I haven't ridden in a while, but the 020's broke in nicely and had a smooth feel all the way to the edge of the tire.
It really sounds like a tire issue...
The other things I can think of off hand is:
a rear load will cause a headshake sometimes (like heavy tail bags).
Did the mechanic check your steering head bearing too ? I could see how a loose steering head would cause issues...
just some ideas, I'm sure you'll get some more responses...
Good Luck,
You'll figure it out, just keep up the good detective work.
Mike :)