I have a 2000 dodge ram 3500 with a cummins. When driving 45-55 mph it stumbles,misses.,when going under or over that speed it runs normal. Is this a transmission problem?

Could be your injector pump.I drive a 1999 2500 Cummins. This pump is well known to cause many problems.Have it diagnosed or it could quit on you.It is an expensive repair.There are many reconditioned pumps on-line.Be sure that is the cause before you spend.The pump is a Bosch VP44.I am not a mechanic but I am dealing with fuel supply issues.Good luck.

If this is a klunk like a split second loss of power then it is fine and it relatively consistantly does it the same way at the same time and under the same conditions and there is NO DTC'S and it is a 45 RFE or 545 RFE or 68 RFE transmission .

had the same problem it was a broken OD accumulator piston spring in the valve body. Just buy a transco SK 45RFE shift kit it comes with all the accumulator piston seals an updated and much thicker and stronger accumulator retainer plate AND the kit includes a "spare" set of accumulator springs (1 large & 1 small) in case the spring(s) are broken ,which they do from timt to time. It is only 6 bolts to remove the valve body (carefully) and 7 screws to remove the accumulator plate to access the accumulator pistons and springs. The instructions included in the kit are excellent and very straight forword, you won't need all of the parts but save them they may come in handy some day later.