Hi, I am a new member and have not found a similar subject in the search function for this forum. Recently, while on a weekend trip in my wife's 2005 Tucson V6, the engine consistently lost power (felt like it was running on 3 or 4 cylinders, but did not backfire) when the automatic downshifted when trying to maintain speed (about 70-75 mph) going up rather small hills. The same power loss occurred at stop signs when accelerating (forcably) away. During these episodes (at each hill), the car would lose about 15-20 mph of speed while going up the hill. If I manually upshifted the automatic, the engine power (all 6 cylinders) would be restored temporarily (until the tranny again downshifted and the power loss again occurred). Today (about 3 days later), before deciding to take the car to the dealer, I tried to replicate the problem but was unable to do it.....the car runs perfectly. As a result, I am hesitant to take it to the dealer since any codes are now likely to be overidden (the car runs fine now and, according to my wife, has for the past 2 days) and if I can't replicate the problem, its unlikely that they will be able to.
Any suggestions?, ideas?, or has any one else had a similar problem that was finally rectified to their satisfaction?

i have had the same problem and have had the veh into the dealership 2x. first time the veh seemed like it was going to stall,stuttered and so i pulled it off the road and had it towed to a closed dealership, spent night in hotel and picked up veh the next morning. was told that problem was water in sparkplugs, or some such nonsense. dont actually remember what crap they told me. when the veh was dropped off, it was raining, alot. but the car broke down hours earlier and 75 miles away and it was bone dry and sunny. oddly enough the veh did the same thing at almost the exact same place on the same interstate. this time with people in the veh and we were able to coax it into a gas station, and fillling it up seemed to take care of the problem. then it happened a third time and at the same place, blah blah blah. the spot is part of a 8hr drive to my father's house and is done almost completly on highways. the spot is high speed 75 mph and somewhat hilly,. and as i fill up before i leave, my tank is low, just hitting the low fuel light. should give me 2 gallons. 2nd dealership gave me crap answers too. said they couldnt read the codes because they have been ovewritten, but maybe it was bad gas!!! apparently only virginia has good gas, and those pesky carolina gas stations are just bad bad bad. sometimes the check engine light has been lit intermittenly, but they cant figure out the problem. went to local auto parts store and they read the code, o2 sensor needed replacing. another dealership concurred and did it. havent had the same problem again, but i never let the tank get that low for high speed driving. we wondered if there was water in the gas, but happened over period of time. wondered if at sustained high speed driving, on hills that the tcson just pulls gas quicker than the system can provide.

any chance you were putting E85 gas in the car in Carolina? they have a few more stations there than in VA...

could be that once the fuel level gets to a certain point, steep incline or decline will make the car think no gas is in the car, and sputter.... (could be why filling the tank "solved" the problem)... just a theory