The problem is mostly occuring when I connect to and after a short time disconnect the board from pc, and try this several times. In software part, there is a timeout check for the communication. So connecting and/or disconnecting the board from PC won't involve some problems in software.

By the way, is there any way to power the MAX 232 directly from the RS-232 line? If there is then it is possible to place some fast optocouplers between MAX 232's TTL outputs and board. So PC and the board will be seperated physically from each other and there won't be any ground loops.