Two days of searching ... thus .. NEVER TRUST THE "PASSIVE" (couple of transistors) LEVEL CONVERTERS!
The level converter I used was something like http://www.pvelectronic.eu/hlavni-odd/podle-kategorii/rs232-ttl-uart/rs232-shifter-smd.html, see the schematics http://www.sparkfun.com/datasheets/Prototyping/RS232-Shifter-SMD.pdf. Don't trust that piece of ... 
At lower speed, or at higher speed/low data volumes it works okay ... but for a data transfer, needed to burn a sketch thru emulated stk programmer (UNO uses 115200bps), it produces some, but only very rare, bit-errors - and you're out of sync.
A simple PL2323 board, robbed from an old mobile phone cord, fixed the problem definitely.
Solved.