Is this happening on all of the 5330e sets or just one or two? If it's happening on all of the 5330e phones, how many phones are in service? What kind of PoE switch(es) are you using at the site? If there is a 5540 console onsite, are the LEDs affected?
I wouldn't think it was a PoE issue either since it's not affecting the displays. But, I'd explore that option by swapping out a spare 5330e if you have one. If you have one at your shop, plug it in and make sure it's fine before taking it to the property. If not, connect one of the phones to a power injector, then plug the phone directly into one of the open LAN ports on the controller. See if that makes a difference.