the mtx75 gearbox is problem free when compared to mmt6. you shouldnt be experiencing this unless the previous owner had issues wth the clutch dragging slightly or was extremely hashy. this can affect your synchromeshes & the demise of the box. have you checked the alignment of your cables? all it takes is a half mm & some gears will be tight/notchy. and changing gearbox oil can sometimes be a bad thing as synchromeshes need friction to operate in these types of boxes. people sometimes dont realise this & think the opposite