I have the same mouseover issue. It's pretty maddening. I know it's not Distance/LoS because I've seen it happen when I'm standing on top of the tank. I'm heals and a I have a real life friend who tanks for me and we've run quite a few dungeons together so he's been helping me diagnose but so far I've found no good solutions. I'm glad I'm always in voice chat with him so I can tell him to burn cooldowns when it happens!