i cannot get to your site, but I have had this issue with the following on a slc 500 over the last 5+ years.
1. the chassis has water / trash in the backplane.
2. corrosion on the backplane.
3. corrosion on the I/o cards at the interface point, reseat several times to clean.
4. bad connection between racks on (2) 1746A10 RACKS.
5. bad connection of the plc to the rack
6. bad power supply connection.
8. bad power supply.
Yes, this is a very dirty / dusty plant and we have had seals to rot on ridged conduit hubs.
james