I feel for ya man!!!!

             I also have had problems with the ball breakin flatline 98c.  1st off paint is definately key with the flatline. My flatline barrel (i have a hunch) is very picky about paint. I think you should watch out for old paint that may sit around for awhile. I can definatlely agree with alexderaven on the dimples and oval shapes. I have used some diablo heat paintballs in the past and they worked pretty good for awhile, but I just bought a new case and the balls looked kind of funny, oval shaped, and a dimple or two. The fourth ball out broke. I have had marbalizers break as well.I'm going to post a poll to find out some reccomendations. 2nd  Try to see where the balls are breaking I did swap my bolt for a rufus dawg bolt system because the balls where breaking at the bolt and that did help alot. I made sure that there are no burs on the barrell adapter or anything that could cause breakage. Make sure the barrel is clean and free from any debris. I wash mine with soap and water and dry it thouroughly. It sucks when a ball breaks and you can't squeegie the barrell clean enough. The barrell becomes useless if its wet with paint and shell bits. 3rd  Make sure your velocity is set well (270-280 fps) recomended for flatlines If you use a cronograph you should check your velocity with a regular barrel and set it up good for 280fps Then attach the flatline making sure it's seated properly then chrono and adjust accordingly. Try not to use a tank thats less than half full or weak. The inside of the barrel had what they call a sand blasted finish it gives the ball traction to gain the backspin it needs. If this surface gets really worn off or chiped it can cause problems, but you can get it resurfaced. Where your barrel is new you should not have this problem but i would check it out. My barrell does have some small scratches from shell bits when the balls break, but i don't think it is the cause a breaking at this point. I still haven't figured out my dilema, but I'll let you know when I do.

Paint is the major key.  A good brand is Xball, and my gun is chronoed at 276 fps.  Having the velocity too high can put too much force on the ball and break it.  Just a couple hints...