if its breaking in the barrel that this is easy.
try some different paint that fits well inside your barrel. we are not looking for accuracy right now, we just want to be able to actually get whole paintballs out of that thing
IF your powertube was cracked, you would experience FPS issues where is jumps all over. sine you started low anyways, i'd really try getting the FPS up higher with a better source than a stretched spring. then if it still jumps around, we can talk internals.
lets not jump all over the place until we solve your origional problems. low FPS (250) and breaking paint.