my car dies as im driving it if there is less than half a tank of gas. what could cause this?

I got my car back from a mechanic less than six months ago. He was working on a cracked head. It has done this ever since I got it back. It sat at the mechanics shop for over a year and a half with a full tank of gas in it. He said he cleaned all the old gas out of the tank but I don't know for sure. But as I'm driving down the road the rpms starts to get lower and it dies.