The issue of the headset coming loose isn't the headset, its the stem. The topcap only helps to set the preload but can't hold everything together, especially since it has nothing solid to connect to; a star nut or plug isn't really that strong. Instead its the stem that holds the works together and you need to make sure you're getting that properly tight, typically 5nm is the torque on the stem bolts but after tightening the second bolt you need to recheck the first bolt you tightened and after double check the second bolt. I've had to do as many as 3 checks as one bolt can lose some tension once the other bolt is tightened. Can also use fibergrip if a carbon steerer to help prevent slipping as well.