The reason to go with a FLIR type is that it will show you the game clearly based on it's own body heat, while a light intensifier does not, just lets you see better.

But that is a LOT of $$$$ to pay. I'd just stop taking late evening shots if in your situation. Early morning would be good to go as would the rest of the day.

Another possible help would be to go to a razor head like the Spitfire or Grim Reaper that give very large cut patterns and will put the deer down quicker.

I hope some of this helps.