WARNING - Balance your ultra light wheels!
At the time I thought it was the weight of the clamp on tri-bars upsetting the bike but it happened again without the tri-bars on. It wasn't a normal speed wobble or 'shimmy' that could be stopped by clamping my knees to the cross bar, it was a natural frequency starting at about 38mph!
It seems obvious now, but it only happened when I had ultra light 38mm carbon wheels on the bike. I had gone at high speed with three other sets of wheels on the bike without a problem. Luckily whilst trying to research the problem I stumbled across a post on a forum from someone with a light Zipp wheel who said that after balancing the wheel it hadn't happened again. I looked on the Zipp website and on the FAQ page it mentions a method for balancing their wheels, so it obviously needs to be done sometimes.
The suggested Zipp method of balancing the wheel is to mix sand and epoxy resin and stick it under the rim tape which sounded rather messy and permanent to me! I weighed the 42mm valve and extension at about 8 grammes. I bought some lead tape which is normally used for adding weight to golf clubs and tennis racquets. It's 12mm wide and 1cm = 1 gramme so I cut 8cm and stuck it opposite the valve, between the spoke holes under the rim tape to balance each wheel.
It took some nerve, but I used the wheels again for the hilly on Saturday (29/08/2015) where I went over 38mph 4 times on the course and have since used them on another bike going over 38mph with no problems at all. In fact, both bikes feel far more stable at high speed than they did before.
My advice is, even if you haven't had a speed wobble, balance your light wheels for a more stable ride at high speed. If you want some lead tape, let me know, as I bought a whole reel of it from a golf shop on ebay for £4.99.