1999 E/B explorer trans slipping 2nd to 3rd shift

I just bought a 1999 Eddie Bauer explorer sohc v6 5 speed auto it has 60,000 miles on it.I bought it off of the original owner.I recently changed plugs,fuel filter,antifreeze,and the trans fluid and filter.I don't think the trans filter was ever changed the fluid was not burnt but was not red either.The explorer has been great for two weeks now,but when i went to tow my snowmobile trailer with one sled on it and two quads total weight 1700 lbs after a half hour of city driving when i took off from a stop the shift from 2nd to 3rd it reved up about 500 rpm only between those two gears like it's slipping.I was wondering what i should be thinking about for repairs.I did talk to one trany place that told me the they would check for codes and then check the line pressure and increase the pressure 10 to 15% something about a ecm problem or dropping the valve body if they have to.Is there a band adjustment on this tranny.Any help or input would be great...Thanx.....

Anyone?
The problem seems to be happening once in a while now.
Today it did it all day for my wife and tonight it never did it once for me.
Could it be a heat thing?Cooler at night so lower trans temps?
Any ideas would be helpful,it's going to the trans shop in the morning.
Thanx......

Well it's fixed? No use in saying what fixed it. Thanks to all that replied to my thread,i'll be visiting this website more often(NOT) :disappoin

Hey man, I have a 1998 4.0 SOHC V6 and my transmission does the same thing! It will slip between 30mph-40mph, no matter if I gun it, or barely give it any acceleration. And, like yours, it does it off and on. I asked my dad's friend, who owns a shop, and he said that he has heard that problem alot. He said that he has heard of some who rebuilt the whole trans. and the problem remained. He has heard it is caused by the computer in the explorer and that people have went to the dealership and they reprogramed it and the problem was fixed. I have not yet went anywhere to try and reprogram it though. But it is so weird, sometimes it shifts perfect and other times it is horrible. I have noticed if you let your foot off the gas it will kick in quicker. Anyways I'm stumped on the whole thing, keep in touch and let me know if you have any ideas on the cause. E-mail is P_71287@yahoo.com.