It could be your brake pads - but i would take it into a mechanic to be sure.

It is not the brake pads.

The most likely cause for a wobble that come and goes at certain speeds is the wheel/tire balance.  One of your wheels probably is probably out of balance, which means that there's more weight at one point of the tire due to uneven tread wear or a slight bend in the wheel.

The easiest solution is to take your car to the local tire shop and ask to check the balance.  Some retailers, such as Les Schwab, will do this for free.

If there is very uneven treadwear,  then you'll need to have the car aligned.  Improper alignment (from hitting speed bumps and holes in the road) will quickly wear out tires.

Another thing to look at is if there are any small cracks or bubbles in the tires.  Again, a good tire retailer can quickly address this -- usually free of charge. 

I agree with sbarer, but just want to mention it may be the disks / rotors (part of the front brakes) as well. It's the same idea as the tyres, they may be out of balance.

To solve this, you'll need to see a mechanic, not just a tyre specialist. As you have a European car, you might wanna look for someone that has experience with this type of car and even it's year / model. Good answer though sbarer!

four possibilitys come into my mind. Most likely is that the tires are out of balance, this would cause more of an over vibration not just steering wheel. Next most likely is a tire that has it's tread separating from the carcase of the tire, it would vibrate mostly the steering wheel. check the tail light sometimes there is something stocked.Last likely is that you have a bent wheel, it would also mostly vibrate the steering wheel but you have to hit something with one of the wheels (like a curb) in order to bend it. Any competent corner tire store should be able to fix this problem without any trouble.