Profibus problem

Hello, we have a problem with a profibus net connected to a S7-300.
The diagnostic software is telling us "I/O error" once in a while, there is no consistency, there is 6 or 7 faults in one minute and then 15 minutes without fault. The fault is detected by the PLC and instantly recover. In the machine it looks like a loss in traction or a slip in the pulleys, but very quick. We already change profibus cards, cable, connectors, check program and reload original programs.
Any ideas??
I would like to try anything new.
Thanks

What does the diagnostic buffer in the PLC say exactly ?
If the profibus is connected via a CP, you can also investigate the diagnostics buffer in the CP.