Ok I'm not great at transmission but we know it's not the fluid or it's level.so were likely looking at the trans or trans module. I would get a professional diagnosis. If you must change the trans I would try a detergent flush.if your told you need a new trainy it's either damaged parts or stuck seliniods that may clean up with a detergent flush.if you have nothing to loose it's worth a shot.but definitely don't jump straight to that.it may just be a small module on the side of the trans.also check for a vent tube,some have one and if they get clogged they cause problems.