If you have a soft comp tire with a high grip rating, you may be developing flatspots. Meaning, you drive to work and park it for 8 hours in the hot sun, the tires (being soft and grippy) get flat spots because they are just sitting there, then when you go to drive at higher speeds (60mph+) the steering wheel will start to vibrate. It happens to me also, I run Michillen Pilot sports, But the tires eventually work the flat spot out so by the time I get home I am running nice and smooth.
However, if you don't have tires like mine, then you might want to check your alignment and wheel balancing.
1999 BMW ///M3