We found one issue in our whole network with port flapping in the log file, when we inspected the situation, we found that it was caused by an old Planet switch. We replaced it with RB2011 in switch mode, and it works great since.
If you have port flapping issue - make sure you try to replace the involved devices, cables and switches. It could be caused by hardware fault, or if the device is old - maybe it was improved in a later revision. For example if you have it on RB450G, check when you bought it, and try a different one (newer) if you can.