Just got a reel of Forten Competition Nylon 15L in black and tried to string my prostaff but the string broke on the machine three times in a row while i was doing the mains at different points. Tension was only 50lb and the grommets seem fine. Had no problem when I put in my usual isospeed professional 17 after giving up on the forten for the day. I know the stuff is dirt cheap and I can't expect much but I ought to at least be able to get it on the racket for a few hours. Has anyone else had a similar experience with this string?