Do this and I bet your welds get better.
Use some scrap pipe to practice on, turn the heat up and slow down the wires speed. Watch the Weld pool as the metal start to melt, as you see the pool forming start to move with a "Zig zag" patter, always make sure the weld pool is there. You will notice if they power is to high if the pool gets big fast (which will create a hole), then just turn the heat down a bit, and keep moving at a even pace around the pipe.
If you have to stop, then stop, move the pipe into position and start again.
Do this and you welds will look smooth.
Oh, always Tack weld your pipe first, It makes it SO MUCH easier when you start to weld.
Also, I know your using Flux core wire, but it is still a good idea to get the base metal clean as possible. Use a wire brush.