intermittent iHistorian Heartbeat alarm

Has any else had issues with the iHistorian heartbeat being intermittent. I'm using a fairly old version of the iHistorian (v2.0). I have the heartbeat configured to write to a iFix SCADA tag with a PLC address. The PLC resets the bit each time the iHist sets it and then starts a timer, which I have set to about 3x the heartbeat interval. If the PLC does not see the heartbeat within that time, then it triggers an alarm bit which is read by an iFix DA block and shows on the operator display.

Recently at a customer site, this alarm has been appearing every few days, it generally clears itself after a certain time...looking at the alarm logs, its usually about 1 hour. When the heartbeat alarm is active, the rest of iFix appears to be working fine...ie it doesn't appear to be a general comm. failure, no ????? on the displays, etc.

I wanted to see if anyone has ever had issues with the heartbeat behaving like this. I do realize this could be an issue on the PLC side or with the comms, but at this point in troubleshooting, I'm trying to decouple the pieces and figure out where to focus, so I figured I'd check here and see if there are any known issues with the heartbeat itself.

I'm no longer actively at this customer's site, so I don't have access to the system right now...but any suggestions for things to look at or try wold be appreciated and I'll test them next time I'm there.

Thanks,

Greg