Just did a search and realized that you posted this same issue 3 years prior (
http://www.peachparts.com/shopforum/tech-help/369583-smaller-water-pump-pulley-m103.html). I would think that unless you can remember a time when you had similar ambient temps and your car never made it past the 85-90°C mark while driving similar speeds then the issue may not be the cooling. You may want to check the tension on the belt, make sure you aren't running lean, and maybe even check to make sure there isn't rust. There must be something that's being missed. We've both had similar issues with cooling and I've also done several mods to the engine to help run it cool.