Hyundai Repair: 2001 Santa Fe Issues, camshaft timing, highway travel

Question
I have a 2001 Santa Fe with just over 136000 kms. In July 2006 I began experiencing some serious "bucking" during highway travel only.  I also started to experience loss of power - top speed was approx 40 km/hour.  Dealership advised I needed a tune up which I went ahead with - plugs and wires were replaced and the problem seemed to be fixed. Next month experienced same issues and was told it must be the timing belt - so that was replaced but I was still experiencing loss of power. The dealership then told me it was the transmission sensor (which they would only guarantee for 2 weeks) so I the sensor replaced.  The issue was fixed for approx 3 weeks & then back to square one again.  Upon returning to the dealership I was told that I would need a new transmission but unfortunately I no longer feel the dealership is being truthful & I feel taken advantage of...I don't really know anything about car repair.  People (incl another mechanic) who I've spoken to have suggested that needing a new transmission is bogus & have suggested that the problem may be the "black box"...granted they've not looked at the car but when more than 5 people say the same thing, I do feel I've been hosed.
Any suggestions would be greatly appreciated.

Answer
There are numerous things that can cause bucking, jerking, and a loss of power.  The fact that the tune up and sensor fixed the problem for a few weeks to a month indicate they were probably the causes of the issue at those times.  

The timing belt is another story.  Not only did it not change the performance of your vehicle, it's also not a prime suspect when the vehicle is bucking at highway speeds.  Furthermore, if the mechanic thought the timing belt was the issue, he should have removed the upper cover and checked the camshaft timing.  If okay, the technician should have moved on to other causes.  The language surrounding the timing belt is troublesome: "must be the timing belt."  This seems to imply it wasn't really checked.  I don't mean to imply that you didn't need a timing belt.  I'm not in a position to know that.  The timing belt is a maintenance item and should be replaced every four years or 100,000 km.  The part I find bothersome is that you were led to believe that the timing belt would fix your problem, and the vehicle was subsequently returned to you without the problem being repaired.  This alone should give you the right to some financial consideration on your repairs if you haven't received it already.

As for the "black box" idea, I doubt that's the issue.  Hyundai has very few significant computer issues.  I know of no known computer programming issues that would be mistaken by a knowledgeable mechanic for an internal transmission issue.  

Needing a transmission is a plausible explanation for what you describe.  And you're in the mileage range where I've seen some transmission failures occur.  But there's just not enough detail in your description for me to narrow the possibilites to a reasonable number.  

If the vehicle is driveable, I'd recommend taking it to another dealer in town for their opinion.  Any way you slice it, replacing or overhauling a transmission is big money.  The charge you'd experience for diagnosis at another dealer is very small compared to the price of the transmission.  I'd view it similarly to insurance.  If the other dealer returns with the same diagnosis, then you should be able to feel confident that the diagnosis is correct.  I'd recommend *not* taking the vehicle to a transmission chain unless the store you take it to is known to be of the highest level of trustworthiness and competence.  It's been my experience that transmission chains like to sell transmissions whether a whole transmission is needed or not.