Rough ride

If the expansion joints repeat at a standard rate and your speed is just right and your wheelbase is just right you might have hit what would be similar to a resonance frequency, In which the spring bounce you get would bounce up and then as its just about down would be hit up again, but this time making it more pronounced and then even bigger and again and over and over. I find this most often on bridges where the expansion joints are more pronounced but have seen it on some highways too. If this is what's happening the only way to fix it is to alter your speed. It will feel like porpoiseing like someone else mentioned.