tonupkid

When you put it into 2nd did you feel it change down? If it did not, there is your problem. Is it possible to get a log og faults out from the car? I don't know, but perhaps someone can help.

As I understand it your LS will hiss when you remove the4 gas cap, because thats the way the car is designed. The fuel tank is intentionally preasurised so as to reduce vapour emmissions into the enviroment. Check what gear your car is in when it goes gutless on you (not by reading the dash) but perhaps by manually downshifting the box or jus observing the upshifts that should be there as your speed increasers (pretty hard on a Lex)

I have another idea for you. How about your auto box is switching into 'get you home mode'. I think this will leave your car in a high gear, whatever speed your going. So, at low speeds you have no power at all, but as your speed picks up to match the 'get you hame' gear setting, eveything appears ok again. Think of it like driving aroung in a manual (stick shift, I think you would call it) stuck in 5th gear. Pretty lousy at pulling off, but once you got up to about 50, everything would seem normal. Switching the car off would give your car the chance to reset (as you said). I once had this exact problem on a GM. Mechanic reckoned it was a stray signal from a spark plug, that corrupted the data in the auto box controller, causing the controller to flip into emergency mode. Changed the plugs and it was fine from then on. This is just a guess, I know sod all about them, but it seems to fit your description. Good luck with it