Kostecki had to out-last a race-long attack from Randle, the gap between the pair barely getting over a second.
At the same time he was complaining of handling issues with his Erebus Camaro.
Still he was able to do enough to win the race and extend his championship lead over Shane van Gisbergen to 111 points.
Kostecki made a lightning start from the outside of the front row, which allowed him to get slightly ahead of Randle on the run to turn 1.
Randle did manage to get back ahead on the way into the corner, but conceded the lead to Kostecki as the track swung left at turn 2.
Behind the lead group there was drama at turn 6 when Jack Le Brocq got into the side of Cam Waters as they argued over 11th place. They then collected 10th-placed Will Davison, the trio all ending up in a synchronised spin.
Davison was able to continue, albeit at the back of the field, while Waters and Le Brocq were both forced into the pits for repairs. Waters lost a lap, Le Brocq three laps.
Back at the front, Kostecki led Randle across the first stint. He couldn't pull away, though, Randle hanging within a second of the leader before taking his mandatory stop on lap 8.
Kostecki covered Randle's stop the following lap, resuming with an unchanged lead.
It was the same story across the second stint, Randle stalking Kostecki at the head of the field.
To complicate matters, Kostecki was also left battling a weird feeling through the steering. Kostecki didn't blink, though, the series leader crossing the line a second ahead of Randle.
"I don't know, it felt really strange [compared to] yesterday and I couldn't brake in a straight line. The car was wandering around a fair bit. It was pretty scary the last 10 laps there, I'm happy to have enough car speed to stay in front of Tom there.
The battle for third was a thriller, largely thanks to an early stop for David Reynolds. The undercut helped him from fifth to third, although meant he would always face an uphill battle with grip late in the race.
He did manage to hang onto the spot until the dying laps, when van Gisbergen made a move at turn 6. However he locked a brake, ran a bit wide and left the door open for Chaz Mostert to take over fourth.
Mostert then ran down Reynolds, easing into third place at turn 1 on the very last lap.
Van Gisbergen, meanwhile, came under fire from Matt Payne on the final lap, but was able to hang on to fifth place.
Payne was sixth ahead of Anton De Pasquale, Bryce Fullwood, Broc Feeney and Andre Heimgartner.
Title contender Will Brown had been in the battle for ninth spot with Feeney, but was shuffled back to 13th late in the race.
The Bend SuperSprint concludes with a final sprint race at 3:50pm local time.
