I was tracking the bug by playing with the MTU for the point-to-point link (smaller packets -> higher probability for the packet to pass the lossy link), by watching the output of ifconfig and by switching kernels. Guess you have to put you hands on the source code for finding the bug (trying kernel.org version of 2.6.15+, 2.6.8; compare them); reported the bug here 'cause it is some kind of Ubuntu issue, but also because I do not care enough for fixing it/reporting it upstream to the right kernel hacker.
>From the MAINTAINERS file: 8250/16?50 (AND CLONE UARTS) SERIAL DRIVER P: Russell King M: [EMAIL PROTECTED] L: linux-serial@vger.kernel.org W: http://serial.sourceforge.net S: Maintained -- High error rate on UMTS modem https://launchpad.net/bugs/45928 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
