Ericsson BP250 caller getting put on hold in middle of conversation 1

[pabx86]

Hi all,I have a very frustrating problem with an Ericsson BP 250 2 cabinet, rel 9 PABX.In the middle of any call the system will intermittently place the called party on hold and they will hear the MOH message. It appears to only occur with cabinet 2 extensions, these extensions are all analog, and the call can be internal from an analog or digital extension, or an external call via the isdn or pstn. It also appears to only occur when there is suspected high useage of the system.I have swapped all the analog extension cards from cabinet 2 into cabinet 1 and the problrm remained with cabinet 2. I then replaced the PUB 3 backplane in cabinet 2 for a PUB 6 backplane. This made no difference so I replaced the powersupply in cabinet 2, line filter for cabinet 2 , interconnecting bridge/bus cable and have connected the system up to a UPS. All to no avail. the system is still putting callers on hold intermittently. Both cabinets are earthed.I have sat down with the software compatability list and all the cards appear to be within the correct ranges.I have completely run out of ideas. The customer had mentioned that the Main processor card had been replaced however I may look at changing this again.My question to you is have you had a similar problem, what did you find and how did you correct it?