I have a 1999 Mazda B4000 V6 with 98000 miles and an automatic transmission. It is sometimes repeatedly jerky when accelerating or going up grade at higher speeds/gears. There is no jerk when changing gears nor any slippage during gear change.There is no abnormal noise. If I step on the gas and drop to a lower gear/higher rpm, it stops jerking until shifting back to the higher gears. The truck drives normally at lower speeds and 1st or 2nd gear. Is this a transmission problem? What is causing it? Do I need a transmission rebuild?