Recently my 98 2500 automatic dodge has been having problems with shifting. when I'm driving about 45-55 mph it shifts up and down constantly. All the fluids are fine, but after i had changed the oil, it was working perfectly. I don't know why it does that and what the problem is. and its also at 309,000 miles everything stock. Don't know if i should replace the tranny or what the problem is. Can someone help??