Hi boet, I have a WR450. It looks like its a common thing with 450s. Mine started to push coolant pass the radiator cap to the reservoir after 30mins of first gear tech riding with lots of clutch use. I dont know if the X has a reservoir, if it does, you can fit a replacement cap that has a higher presure release or you can just fit a fan with a manual switch to turn it on once it seems to get too hot. I used a genuine Yamaha fan from a YZF-R150 road bike, its cheap, works great and doesnt have a big current draw on the electrical system. Also try some brands of engine coolants such as ENGINE-ICE or equal. The fan is the only lasting salution if your planning to overheat it regularly.