99 F-450 7.3 4x4 automatic trans problems

[12vtoddhartpsd]

bought a 99 f450 last week with 245,000 miles it ran perfect except for an exaust leak on the up pipes changed all fluids and filters trans fluid, oil and filter, fuel filter, front end and rear end, it shifted fine except 3rd at 30mph if i let out of it and slowly get back in it it wil down shift and up shift real quick then the other day i was crusing at 40mph and it shifted down then back up real quick it has done this about 5 times in one week. trans fluid is clean and at the right level. i had a edge down loader on it set on tow when my truck did it the first time i took it off but the trans is still doing it and it will only do it under light throttle. any help trouble shooting this? it has 4.56 gears