I had been using Parallels 17 to play "Battle Academy" (a Steam game) and it worked perfectly fine. Then I "upgraded" to Parallels 18 and, when playing the game, the screen flickered so badly I had to stop. I tried everything (RAM allocation, screen resolution,...) and nothing alleviated the problem. I uninstalled 18, re-installed 17, and it works perfectly again. Whatever 18 did it ruined my ability to play that game. (the rest of the Windows experience is fine).