My experience is that some pilots don't fully understand the true significance of a sudden outside air temperature rise (and yes they do occur - I have seen temperature rises of 20-30 degrees).
When we are flying at a constant Flight Level we are in fact flying at a constant 'indicated' altitude. The true height of this Flight Level above mean sea level (AMSL) will vary with the outside air temperature. In cold air it is lower, in warm air it is higher. So when we encounter a sudden increase in outside air temperature, to maintain a constant Flight Level the aircraft has to increase it's true altitude AMSL. ie it has climb against gravity, (but in the flight deck the altimeter and VSI will still show level flight because we are flying a constant Flight Level ie 'indicated' altitude.)
Now to climb against gravity we have to add energy and this ideally comes from the engines increasing thrust. Where the problem arises is if we are cruising near max altitude and the aircraft is cruise thrust limited. If the engines are at max thrust, they can't add any more energy. With the autopilot maintaining a constant Flight Level, but the true altitude increasing, the only available source of energy is speed. ie speed is converted into height and the aircraft's speed reduces. And as has been mentioned before, if you're in 'coffin corner' a stall is not too far away. The only solution is to descend and you may not have much time to make that descision.