I am also experiencing this issue. Just bought my early 2015 MacBook Air 11.6" last week and attempted to install Windows 10 via Bootcamp using the newest Bootcamp software (6.0.6136) through El Capitan. I used the newest Windows 10 ISO directly from Microsoft's media creation tool. The installation completes without any issues, but after installing the Bootcamp software and restarting Windows, I am getting the flickering, horizontal lines and artifacting that is described above. I have tried every Intel HD 6000 graphics driver for Windows 10 that is offered by Intel, but the issue persists. Some drivers are worse than others, but all exhibit the same symptoms at some level. I have been able to achieve 90% stability using driver version 20.19.15.4444, which is currently the newest driver available from Intel.
I called Apple support to report/troubleshoot this, but was told that "Apple doesn't write the drivers, so take it up with Microsoft Support." That's complete BS, seeing as the package containing the drivers comes directly from Apple, packaged as Bootcamp and Windows support software for this specific MacBook. Apparently, there is no real testing going on prior to releasing these new Bootcamp versions, leading to a bad user experience for myself and many others. There are 3 reasons I bought a Mac: 1. Aesthetic appeal 2. Build Quality 3. The ability to run Mac OS X and Windows natively on a single machine. The fact that running Windows is really unpleasant due to the display issues, I am leaning towards returning the laptop for a refund. Maybe in time there will be a driver that offers flawless operation, but then again, maybe there won't be.