RE: Vertical Stall
Whatever the terminology it boils down to your engine is not producing enough thrust to pull the plane vertical. The easiest solution is to check your tuning.
Also, did you change props to a different size or manufacturer?
Does the model weigh more because you installed a larger battery or.....?
Are you using the same fuel or is it old/moisture?
As already mentioned, the cooler weather will make a difference in tuning and therefore power of the engine. As also mentioned there are fuel problems that can contribute to loss of power. Also, needles have vibrated out of adjustment, clunks move and stick forward in the tank.
Give your plane/engine a good check.
I had a fairly recent battle with an engine. The plane flew ok and had power, but when I finally got it nailed down I was rewarded with a "hammer the throttle and unlimited vertical". The plane shouldn't do it with the combination, but she sure proved me wrong! Sorry, got a little excited there.
Good luck