**Problem is now resolved for us**
We found the following in the Pcoip server logs and found many many out of order packets of UDP/pcoip traffic.:
IPC :Received ACK on fsp 6 for unknown flight seq 62
IPC :Received ACK on fsp 11 for unknown flight seq 58
We then switched the Local sites switch stack (Master) switch to Slave and allowed 1 of the other switches to become (Master)
And immediately there was no "out of order" packets and the Pcoip log files did not show the "unknown flight" errors i pasted above anymore.
As a result we replaced the origonal master switch and have a very happy office.
hope this helps someone else.
Regards