1998 Dodge Ram Buckiing/coughing?

Hello, I just bought a great used 1998 Dodge Ram 1500 that has had a substantial amount of work done to it.. right before I bought it, the owner had a $950 tune up, oil changed, emission tested and inspected. Everything passed. The only thing the owner said was it might buck and cough for a little and then will run smooth.. Well it does exactly that..I will start it up right away..no problems..Then I will drive it about 2 miles maybe less and it will buck/cough pretty hard to the point I can only give gas and then let go of the gas and idle down the street..I continue this for 5 minutes and then it picks up and it drive like brand new..I drove it about 300 miles to new jersey and back to connecticut and it was so smooth and a great ride besides the bucking once a day.. maybe transmition, fuel injectors?? please help me!