personally I doubt that youwill solve this by increasing the cycle time of the CPU. The problem is rather to be found at the bus, I guess... but it is only a guess.
Sometimes sporadic problems appear also in industrial RS 485 bus systems... even with good grounding. To solve this I would recommend to get a specialist on site with adequate equipment to measure your network and to keep an eye on it until the problem appears.

If you are lucky you will also find certain relations between the appearing problems and other events on site. As I remember... even passing trains created some problems on a bus system once... but this is a long time ago.

Best regards