Serial Channel: End of transmission

Because of the not working TXBC I’m trying to recognize the end of a transmission by receiving the data stream I’m transmitting. On my hardware the serial interface is connected to a RS485 driver and when you enable the receiver while you are transmitting (both directions are enabled), you have the same data stream on TXD and RXD. On the NET+50 I enabled the Receive Register Ready Interrupt and the Receive Buffer Closed Interrupt. But as my stream of characters (6-18 bytes adjoining each other) is not (often) divisible by 4 without remainder I always lose the last 1-3 characters. As described in this forum I tried to solve the problem by using the Receive Character Gap Timer but this caused even more problems. The Character Gap Timer Interrupt appeared before the gap appeared and so I even lost more characters. Does anybody know, if the problem is caused by the simultaneously transmission and reception or is there another not published field which does not working correctly?