Do you have particle effects on? Some infernals are invisable, the only way to see them is with particle effects
Have you tried using the Thaumcraft or AM2 true sight effects, or the witchery brew of revealing on them?
No, but they will probably have no effect. The thaumcraft one however I can try. Maybe they spawn with armor with that invis enchant and that messes them up?
Yea, I had the water guardian do this to me. Quick log in and out seemed to fix it though. Figured it was just on my end with such an old pc.