I have a 2004 Santa Fe with 150,000 miles on it. The issue is when trying to run at a steady speed (40, 45, 55 mph, etc) all of a sudden without warning it will drop 500 RPM or more. It then catches itself and picks back up and runs for awhile with no issues. It never completely dies, at least it hasn't as of yet. It could be fine for an hour or two then starts doing this continuously for 5 to 10 minutes. Then it runs fine for another little bit of time. The coil packs are brand new (just changed them today). It pulls from a stop with plenty of power and no hesitation. It idles fine right around 500 RPM. It doesn't stall or backfire. I hope someone out there can over some suggestions before I condemn what is otherwise a great car and go shopping for something I really can't afford!

 

Does the speed remain the same or does it drop along with the rpm. It may be the transmission shifting to overdrive (decrease in rpm) then out of OD (rpm increase) while speed remains ~the same.

 

The speed drops too. It never goes all the way down to zero either on the tach or the speedometer. The best way I can describe it, it just looses power, the RPM drops, the car slows and then it "seems" to remember what it was doing and goes back to the speed it was at. I don't think that it's the transmission because of the varying speeds at which it happens and I can run the transmission up and down manually without an issue. Maybe it's somehow running out of fuel? If it where an electrical malfunction I would think it would throw a code, but nothing there. I'm stumped and just don't want to start throwing parts at it. If anyone has had this problem please let me know because I sure can't go anywhere with the car like this