It sounds to me like maybe you're letting the sled just ride on the snow when you turn out. If you're climbing and losing momentum and turning out to the right, put your weight on the left (uphill) side of the machine and pull up on the right side while staying on the throttle. If you have a strap in the center you can use your left hand to help pull the sled up too. Turning out at the top is just like sidehilling in my opinion. If you practice sidehilling in small bowls or just on a little hill, turning out at the top will become second nature.
What he said^^^^. sounds like you are cutting the throttle going across the hill. The track needs to stay spinning in order to cut into the sidehill, if you just drop the throttle when you get turned sideways across the hill, the track speed will slow and the track will try and "lay down" onto the hill. At this point you are never going to save it, once the sled is onto the negative angle on the downhill side, its like laying it on its side on flat ground and trying to pull it upright by standing on the running board.
Its a balance, you have to stay in it long enough so that when you are trying to turn out of it you are not turning out at 30 mph and not staying in it too long where the sled stops, flips backwards or you don't have enough momentum to turn out. Start on a small, wide open hill in the powder and head up it, slow down if you can actually make it over the hill, then gun the throttle and turn out, keeping the power on until you are about 3/4 of the way pointing straight down. You only need to keep the power on until you are clear of the sideways part of the turn, otherwise you end up travelling across the face of the slope and you will find that you have travelled a great distance across the slope (almost sidehilling) which can get you into serious trouble if you didn't plan to come down the hill in that area. After each run on the small hill, stop at the bottom (just momentarily - can leave the sled running) and look at your line. Think about things like how sharp the top of your turn is, when you turned out how close you were to not making it, etc. If you see your down line trails way off to the right or left, you stayed in the throttle too long in the turn. If you rolled it, you didnt' stay in it long enough or you didn't have enough momentum. Then do it again, using the other mark as reference, if you got stuck or rolled, then turn out 50 ft sooner (or 100 ft sooner depending on the hill etc.) and see how it feels. Another key is to learn to shift weight fast, think and react fast, move fast and turn out fast. Ability to realise when you aren't going to make it and make a quick decision to turn out will really help. The momentary delay in that decision can mean rolling it down or riding it down.
Always read a hill before you ride it, plan a route down so that if you have to turn out quickly, you don't turn to the left/right where that big rock or cliff is. Often snow conditions change based on hill parameters. Last year I was riding up a big open slope in about 2 ft of powder with a very good base, went to squeeze through a 30 ft wide rock opening about 30 ft long (no big deal) but the snow had not settled between the rocks like the rest of the area and the base just went away on me, it was almost 65 degrees steep but it was such a short run I thought I would make it no probs, had enough speed when I entered it. Anyway, trenched right down and sled wheelied right over backwards on me in the first 15 ft
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. Right over backwards and it drove itself to the bottom. The conditions can change that fast.