LabVIEW RT, Timed loop, finished late, Call by reference

Question

I have a timed loop triggered by the sample clock of a DAQ-Card. The sample Clock is 8 kHz and the loop will run with dt = 4. Normally the loop is running without finished late[i-1]. But from time to time it happens that the loop is running extremly longer which means instead of 0.5 millisec it needs 4 - 5 millisec. It seems this doesn't never occur while accessing DAQmx.
The application uses plugin technologies and some of the VIs in the timed loop are preloaded and called by Call by Reference.
Does those VIs inherit the priority, execution system and CPU of the timed loop?
The application is running on LV RT 2009 on a Dual core PXI-Controller. The timed loop is bound to CPU 1. There is no  difference runinng the application from the development environment or as a startup application.
For the measuring test  I modified the application in a way that I don't have:
disk access for storing the result file
TCP/IP communication
Controls on the front panel of the top level VI
Waldemar
Using 7.1.1, 8.5.1, 8.6.1, 2009 on XP and RT
Don't forget to give Kudos to good answers and/or questions

Answer 1

To keep you informed:
I stripped the application and have left only the measurement storing module and the timed loop. The loop was using the microsecond timer running each 500 µsec. Additional all Non-Real-Time modules were loaded and only one module creates a XML string and sends it to the communication layer which will drop it because no TCP/IP port is open. The creation of the XML string is done each 300 ms.
In this case I don't have any finished late iterations
Next I added the DAQ Module. Now I get finished late again but with a lesser frequency as original.
I removed all unnesseccary tasks from the DAQ moulde leaving one task for a PXI-4204 for using as the clock source for the timed loop. No I get finished late seldom.
 I removed the module which will send the XML string and I don't get finished late.
Next I was adding code to see memory allocation. I can when memory allocation is changing but it is not related to the finished late iterations.
Next time I have the chance to do more tests I will see which DAQ task triggers the finished late iterations. I have one AI task on a PXI-4204,  4 Counter tasks on a PXI-6602, 1 DI task on a PXI-6514, 1 DO task on a PXI-6514, 1 DI task on a PXI-6259, 1 DO task on a PXI-6259, 1 AO task on a PXI-6259 and 1 AO task on a PXI-6711.
The AI task on the PXI-4204 is running in Continous Sampling (Single Point HW Timed is not supported), all other tasks exept the DI and DO are Single Point HW Timed.
Waldemar
Using 7.1.1, 8.5.1, 8.6.1, 2009 on XP and RT
Don't forget to give Kudos to good answers and/or questions