07 tranny problem?

ive got an 07 5.9 mega cab dually. no engine codes. my truck is having trouble shifting from 1-2 i believe (around 30km/p) it holds the gear and then finally goes into the next itll slip a little and then be fine thru all the other gears. happend once and a while. i have recently changed my tipum and have one in now that is "kinda rigged" to work lol. could this cause a problem? thanks.