Holding prime rib

Ive got an event at work coming up where we will be cooking 5 prime rib loins on pellet grills. easy to cook, yesterday I cooked one for practice, pulled it at 126 and put straight into a cooler, over the next 2 1/2 hours the temp slowly climbed to a maximum of 149. so that didn't work, how can I cook prime rib and hold for a couple hours with out that happening.. PS after 5 hours in the cooler is was still 138, thanks for the advice

I went with what worked over the weekend. Had 8 full rib loins to cook and 6 pellet grills. started cooking at 9:30, pulled each one when it hit 115, about (3.5 hours at 325) foil over the pan and into a cooler. temps climbed to 135 to 140. was still 130 at 5 when we started serving. cooked the last two loins after the first 6, couldn't tell the difference. all good.