Hi I own a 2000 volvo s80 2.9 w 96,000 miles and recently I noticed the car is surging on the freeway when its in overdrive its about a 2-300rpm differance when it happens you slightly feel it while being drivin. Now when I shut the STC switch off the car is pefect on the freeway ! I gave the car an inspection everthing in the front suspention is tight the trans fluid looks/smells good but Im gonna change the filter and fluid anyway I have no clue what could be going on! CAN SOMEONE HELP?

Changed trans fluid and filter, car still continues with same issue and now noticing antifreeze scent in car.