Strange happening with onmonitor

Yesterday I observed something really strange and wondered if any of you had ever seen anything similar. We had been running 7.30 UC6?(it may have been 7 - i'm not on site any more to check it out) on a number of SGI boxes. We used onmonitor successfully in the morning but towards the middle of the afternoon we kept getting memory-fault core dumped, and bus error core dumped messages from nearly every facility of onmonitor including displays. The system had been installed for a long time with no problems and the instance we were running had been up for 57 days. The normal user uses onmonitor quite a lot. We tried out 2 other machines, one with the same hardware and one with a different processor. All 3 systems had the same problem. Everything else worked normally and onstat was running OK. We tried shutting down an instance and bringing it up again. Still no joy. Eventually we decided to re-install the software from the distribution media. The problem went away on one of the machines. As the other machines are in "pseudo production" we are unable to confirm that the same solution would work elsewhere. Your views would be much appreciated. regards Malcolm