serial issues on fast PC

Hi, While testing on a new PC I ran into the following issue. Same phone (C115), same USB cable FTDI RS232 + RS232 -> 3v3, works on a slow Atom PC with latest osmocom-bb git, but fails with fmtool error on a fast i7 core PC. The fail happens after sending the first 4096 bytes with a NACK from the phone. Once in many tries it actually does download something but then fails to run the image. Has anyone seen such behaviour? Any ideas on what might go wrong? If not I'll spend some time to debug it. Best regards, Job