904 flare-up during 2-3 shift

Well the areated oil isn't a good thing to see. That could be causing it. Maybe they missed a bolt torqueing down the valve body or didn't tighten the filter good. Another thing to check is the throttle pressure (kickdown) linkage adjustment. If it's not adjusted correctly it will cause flare ups on shifting. Was any type of shift improver kit installed when they rebuilt it?