yep sounds like she's heating up. When towing is she hunting between 3rd and 4th? Reason I ask is as the trans shifts it slips to cushion the shifts which when it slips it generates excess heat. Sometimes it's better to just hold her in 3rd gear or speed up so that she's not hunting.
extra cooling is a must with any auto trans. I would pick up like Mel suggested a fan cooled unit or get the biggest thing you can lay yer hands on and mount it in front of the ac condenser, just plumb it in so that it's the very last thing the fluid sees before the trans. The rear line on the trans is the RETURN line the front line is the HOT OUTPUT line to the stock cooler in the bottom of the rad. that is what I ran on the E and my trans temps never got out of line unless I was backing up the driveway (uphill) to put the trailer away.