The seals in the caps shrink out of size after a while. Usually along one edge first and no amount of tightening stops it leaking. I have personally found best results by pulling the rubber seal out and running plastic on plastic. Seals next to perfect. Have to make sure the top of the tank is level though.
Good review Mazi. Needs pics
You'll probably find switching tyres will make a big difference, as does air pressure across the brands.