serial jitter

Hello,

I want to send serial data via RS485 port every 2ms on PXI 8433 board with 8106 controller.  Baud rate: 2Mbit.

I used timed loop with loop period is 2ms. I monitored loop time intervals with a chart. When I send 8 byte data it causes to jitter and loop time increases to any value between 12-40ms with nearly 500- 800ms time intervals.

But when I send 7 byte data loop continously run without jitter.

I changed Baud Rate, transmiter buffer size, increased looptime up to 10ms, sometimes jitter interval or loop time changed but never run exact time intervals which I set.

Why this problem has occured? I appended sample project file and JPEG images of 7Byte and 8Byte looptime charts.

Thanks.