Random number of iDocs going missing

Hi,

A client is experiencing some issues on their QAS systems with a random number of iDocs going missing during the transfer from ECC to PI.

I have read through countless discussions already and I'm drawing a blank.

The setup is as follows:

PI QAS System:  Windows/Oracle (11.2.0).  Netweaver  731.

ECC QAS System:  Windows/Oracle (11.2.0).  ECC 6.0 EPH2.  Netweaver 7.0 SP29

I have a job I can run with a variant which attempts to transfer 500 iDocs from ECC to PI.

On the last attempt 162 were found on PI after the job ran.  The time previous to that, 197 were found on PI.

Status of all iDocs in ECC = 3

SMQ1/SMQ2/SM58 have been checked - all clear.

SM21 is swapped with warnings though.  I've attached an example. 

I'm assuming the RFC is dropping out part way through the process maybe??  SM59 tests all are fine.

Any suggestions on where to look next would be greatly appreciated.

Thanks