My husband and I both drive 2007 Sonatas. Only difference is mine is red and his is white. Both have 4 cyl. engines and auto trannies. His had 64K miles and mine has 50K miles. About 35K on his, it began to "surge and jerk" at 46 mph to 51 mph. Nothing was found. Changed tranny fluid and it stopped for a bit. Now it's back. Now my car is in the shop for the same reason. Cannot find a code, cannot reproduce the problem. Told service guy to drive it where I drive it every day and he'd reproduce it. Tach is always at 1500 to 2000 and never fluctuates when it happens. Torque convertor was mentioned to us by another mechanic, but dealer doesn't think that's it. Also, someone suggesed harmonic balancer, but that was a new one on me. Hyundai did start a ticket on hubby's car, but no word. Any suggestions? Car with dealer until Friday.....Thanks!