Response of serial connection is messed up sometimes

Hello,

I have a simple serial connection (ModBus) to read out 2 kinds of data each second (see attachment "VI.jpg"). This works fine but sometimes the response Hex strings are messed up (to less bytes or to much bytes) which results in "ugly" peaks as shown in the attachment "Chart.jpg". 

Maybe its just a timing issue. There are a lot of LabView examples about serial connections out there and some of them has a "wait" function before R/W operations and some doesn't... Maybe you can help me to get rid of the messed responses.

Thanks in advance

ChriKo