shifting problems

Hey, my brother has an 89 dakota sport 4x4 3.9l V6 automatic. Well we went off roading the other day and he ran into some problems. the truck had the tendancy to back fire and miss once the tach hits 3000rpm, the truck wont upshift past this point, if you drive along slow it will shift fine,m but if you put your foot down it will down shift and the tach climbs to 3 grand and just stays there, wont rev any higher and wont shift. I need help with this one, not that familular with dodges but i need to fix his truck for him. Also when its cold it startsd like shit, if you dont give it gas it will sputter and die. He has changed the heat riser valve, plugs, wires, cap and rotor. Could this shifting/back firing problem be as stupid as the coil? Come on guys I need your inputs and help.

Thanks
Ryan