Car stalls and shuts off when I stop, and or turn sometimes at low speed.

I own a 2005 Cadillac SRX, 6cyl, with 58,000 miles. It has suddenly started to sputter, loose power and stall out at random. If I shut it off and restart it, it will drive normally for awhile and then do it again, at random. Also does this sometimes when I turn corners at low speeds. It also sometimes shifts gears violently. No check engine light, I have taken it in to the dealership 3 times. No answers. Had battery replaced now the problem is worse. Just had engine tune up. Problems where occurring before tune up. Had this issue for over a month now. Please help