You can kinda quick clean it with a cotton swab (just slide a fluffy swab up in there), but to get it completely i dont know of any other way then to take it off and run a squeege through it.
And the balls going any direction can be any of three things:
1. Velocity too high or too low... you need to play with this one (Most likely this is not the problem)
2. Ball to bore size... this a huge problem especially since the Flatline barrells are bers picky when it comes to paint. Make sure the paint you are using is .68 cal (I believe that is the Flatlin bore size)
3. Quality of the ball... the smoother it is the better! No seams is the best... make sure there are no dimples, there not egg shaped, or anything else thats i effecting the roundness of the ball. Flatline's NEED almost if not perfectly round paintballs to function properly.
Just play around with all these factors and everntually you will get it right! Good luck I hope I helped... and if i got anything wrong you posted in the right place to get a good answer!