I had the same problem and could virtually guarantee it whenever I went on a longish run. I followed the advice that Mike points to, but only did the first bit - removing the breather from the crank-case to the catch-can and refitting it the other way round. I found that this part was all I needed.
I also began to make sure that I didn't put too much oil into the engine. I keep the level just below the half-way mark and use an energy drink bottle with a nozzle for topping up. This lets you put in small amounts at a time until you've got the level you want.
Tom