Bumpy Roads!
It's all down to construction. When the Thais built roads in the past, still do, they didn't use stone as a sub base. They just compacted soil and Tarmacked over it. Consequently rainwater finds its way through and the sub base becomes spongy and then you get the problem. What they now do is make a concreted sub base slab. Give it time and all roads will be built this way in Thailand, hopefully.