I just purchased a 2004 XJ with 47,000 miles on the clock and am experiencing something odd. When driving and the car shifts into the final gear or convertor lock between 4o-50 mph., if you watch the tachometer, it jumps back and forth from 1500rpm to 1700 rpm until you ease off the throttle or accelerate a bit more in which case it smooths out. When checked for any codes, it shows none malfunctioning I figured since this car had so little miles, never wrecked or damaged in any way, that possibly the trans fluid may have gummed up and caused maybe the convertor lock up solenoid to malfunction. Changed fluid with correct replacement fluid, no difference. Had computer re-flashed, no difference. Had local transmission specialist replace solenoid, no difference. Now I`m really stumped. Thought maybe throttle positioning sensor might be bad, but again no codes showing even while driving and problem is occurring. Has anyone else experienced this problem or know the fix? What am I overlooking? Please Help.