On Tippmanns, you have to REALLY clean the whole setup to get ball flight right again. Run the barrel under warm water, making sure to get the paint out of the rifling holes, clean the entire inside of the marker as paint will seep all over the internals, clean the elbow, and even the neck of the hopper, as paint will sometimes fly up there.
If you on the field, well, sqweege the barrel as best as you can, and just play real up tight with your opponent so the erratic pattern isnt a problem.