IMHO, it sounds like you're running it too lean. When the nose is pointing up it has to strain to get fuel, so it goes lean. That's why it gets better when you lower the nose. As far as the temperature thing, that makes sense too: warm air is less dense than cold air. So for a given needle valve setting, flying in warmer weather (warmer air) means there's "less air" per a unit volume of air (less dense), making you're mixture more rich than in colder weather. All this plus the fact 4 strokes seem to like being run rich. I've got a Saito .91 on a Doghouse eXtreme, and it almost has to be gurgling to perform well vertically - what sounds about right for "normal" flying doesn't seem right for hovering. But it's got plenty of power so running rich doesn't affect normal flight, and when you point the nose up it leans just a little and runs perfectly. So just try running 1/8 to 1/4 turn richer next time, and see how the engine responds. BTW, what prop are you running? I've been using a APC 14x4W with good success.

Good luck!!!
-Matt