2001 tracker surges at 40-45mph

My brother-in-law has a Tracker (V6) that surges at 40 - 45mph. Dealer had the car several weeks. Replaced engine chip, trans., plugs, brakes, hal shafts... still surges. Warranty is covering the bulk of the repairs but no solution on the horizon. Anyone have a similar problem or better yet a fix?

Thanks,
Ray R.