This happened about 4 or 5 times. Going up hill about 45 mph, eased off of the gas near the top of the hill and it felt like the transmission or torque converter was having a hard time shifting up or locking up, causing a shuddering. Let completely off the gas and it stopped shuddering and was ok.

When not under power, going down hill or coasting at a very low speed, there is a very hard clunk that the mechanics think is the transmission shifting. When the mechanic put it into manual low gear this does not happen. Two separate dealerships said it is just shifting and nothing is wrong. The problem happened under the warranty period but they won't do anything about it.