Surging Sonata

The car runs smooth when idling, runs smooth when accelerating, but as the car gets close to the desired speed, it starts dropping rpms, then surging
back up again. It's really hard to drive because the car is continously jerking.
With the cruise set at about 73mph, it was running at about 2900rpms then it was hesitate the rpms would drop to about 2500rpms, it would surge back up to about 3100 rpms and then level out to about 2900 rpms. The intervals between it doing this would depend on the road (going up/down hills or flat surface)I would guess at least every 45 - 90 seconds. (Note: It does NOT matter what the speed I'm driving at, it continously does this. Also, the car will not shift gears until it reaches about 4000rpms regardless of the speed of the vehicle)
I recently replaced the throttle position sensor (28 August 07) and the throttle body (22 September 07. After I replaced the throttle position sensor, it stopped surging for 1 day, after I replaced the throttle body, the car ran great for about three weeks, then it started surging again.
Could anyone offer any assistance or advise to resolve this problem? I would really appreicate it.
Tks, Mark
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It would help if we knew a little more about your Sonata, What year? What motor? What kind of transmission?
Rob

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Have you scanned this car yet? Sounds like the lock up is acting up, but I'm wondering what's going on with the high shift points.
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Like Mike said, it sounds like the lock up converter is going in and out. It used to be that cars had a vacuum actuator for that function and sometimes the line would leak. How many miles are on it and what year? It could also be a plugged catalytic converter which would cause you to use a lot more throttle and the transmission may think you want to downshift. The easiest way to check that is to put a vacuum gauge on the intake manifold somewhere and see if the vacuum is up around 27 inches or so or down a lot lower. If it is real low, your cat is plugged.
Tom

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Tom,
Thanks for the information. I'll have it scanned again this weekend and get the codes. I'll also research the Cat/Vacuum route.
Tks, Mark
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Mark:
Your comments are automatically posting to alt.autos.hyundai (usenet) and without you including the text that you are replying to it is difficult to figure out the context of your replies. Please include the text you are replying to so we can keep up with the conversation.
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Mike,
I had to scanned in Miami at the end of August (30th), which showed throttle position sensor was bad. It was replaced them and the throttle body was replaced about 3 weeks ago.
I'm having scanned again this weekend, I'll update this on Saturday or Sunday.
Tks, Mark
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Rob,
It's a 2004 with a 2.7L V6 and automatic transmission.
Tks, Mark
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