Exchange email reception problem, mail profile stuck

We have several dozens of iPhone 5S and 6, working on iOS7 and iOS8.

All iPhones use the built-in mail application with a mail profile. Email system is Office365 (exchange type).

We found a problem on 2 devices with iOS8 (one is a 5S, the other 6). The problem is present on a iOS 8.1.3 but was already observed on 8.0.2).

Users receive emails by batch once every 4-5 hours or more, instead of immediately, the device keeps trying to sync emails with no success, processor is heating (and causes battery drain).

In some cases, during the different attemps, there was also a Gmail profile, working correctly.

If we try to disable some features in the mail profile (like contacts or calendar); the device starts deleting but in fact keeps deleting with no end.

We tried to remove all possible other applications, including gmail profile, but the problem re-appears.

We tried to change from Push to Fetch or manual, no change

We tried hard reboot, sync only via 3/4G, reset all network settings, no change.

We tried to switch the device with other people, and the result is that it is not an hardware problem.

As a temporary fix, we can completely reset the device and reconfigure from scratch. It can work some days and then suddenly the problem may reappear.

I suspect a data received from server corrupts the local device's database.

Anyone else found a similar problem?

Anyone else found a solution?