This sounds like you're running lean when you're accelerating. If
the acceleration enrichment function of your fuel metering system isn't
working right, the car will stutter, buck, and backfire if you quickly
open the throttle. Once the steady-state part of the metering system
is all that's required, the roughness will go away.

Try cleaning the contacts on the throttle trigger points (later FI
systems call this a throttle position sensor but the D-jet system
doesn't pay attention to position, only changes in state).
chris