Tranny problems in 99 Ram with 46 RE 4spd auto

[Blown68Coronet]

My tranny in my 99 Ram is acting up and am looking for input as to what it might possibly be. When taking off it stays in 1st and won't up shift, sometimes it will if i put it in neutral for a couple seconds and back in drive then it "sometimes" shifts up. I see these are notorious for problems and wonder if anyone else has had similair problems, i changed the fluid/filter a couple days ago when it started acting up, after a while the O/D will kick off and the tranny temp light will come on as well. Also when i checked the cooler lines the lines are cool but the tranny is hot, wondering what could be the culprit?

[Heck2G14]

I had the same exact thing going on with my 99 ram.  I was told at that time the slipping was due to the ATF+3 tranny fluid gums up on shift sensor and gives false shift indication.I did have my tranny flushed and also conditioner ran through it to clean gummed up sensor. Also had a new filter and ATF+4 tranny oil placed in it. This did the trick.  I addressed the issue as soon as it started slipping between gears and not upshifting at proper RPM.  Very few people will recommend a tranny flush because most service deptartments use inproper amounts of fluids during flush and cause more damage.  I did not learn that until after the fact. My truck now has no slippage in cold and very little (barely noticeable) when it gets below freezing.  I am unsure if that slippage is due to the flush or just damage done before the flush was done to mine.