In case the real culprit isnt found, then you could consider to drop the speed from 1.5M to 187.5k. At 187.5k, Profibus is much more forgiving. Of course the TTR increases accordingly, but in many applications that is not so important.
If the above does not help, try to switch the Profibus Profile to User Defined, and then increase Retry Limit from 1 to 3. That will make the network much more resilient against noise, but it does mean it can freeze for 3xTTR when it encounters bus problems.