1990 Dodge Ram Transshifting problems

Hello my name is james and I have a 1990 dodge ram D150 thats been in my family pretty much as long as I can remember, my problem is i'm having tranny issues, i'v changed the tranny fluid and filter etc also changed the speed sensor, i'm still having problems with it shifting, it will hammer shift from 1st to 2nd then it will take forever shifting into 3rd and will rarely shift into 4th, BUT when it does shift into 4th the speedometer works and every thing runs great then all of the sudden it will downsift back into 3rd then the speedometer will cut off, also it will go crazy when I put it into reverse, it will try and rev up and I have to pretty much stand on the brake to stop it then it will stall, it will do that over an over again until I put it into drive, once I put it into drive the idle levels out, then when I pull up to a stop light it wont down shift into 1st and if I dont manually donwnshift and kinda keep my foot on the gas while i'm braking it will stall, I use the truck for work every day and I dont want to see it goto the scrap yard because of something simple but I also dont want to spend a lot of money on a new tranny. I am going to try and replace the speed sensor once again and see if maybe they sold me the wrong one or maybe they sold me a dud. But if you have any suggestions on how I can fix the problem or point me in the right direction i'd be very thankful!