Factory rebuild? Driveway rebuild?  The "when cold"  symptom makes it
sound like a dirty valve body.Was the VB rebuilt? I would use Seafoam
Trans tune.Get it at advance autoparts. This is not a seal conditioner,
like all the other products on the shelf(you may want to add a seal
conditioner if this works). Pour it in and step back.You may want to drain
a bit of fluid ,first. Loosen the lock nut where the fill tube enters the
trans, and let some fluid drain.You definetly will want to drain it
afterwards, as red fluid will now be balck.Maybe it will take two or three
drainings.I used this on my daughters car. I drained it twice.Two weeks
later  she took her car to a shop for something else,and they said the
fluid was dirty.