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Brian ,
certianly shielded cables should always be used , this may well be your problem , use good screened cable try and keep the RJ12 close to the sheath but do not connect the shield at this end , make sure the sheild goes right back to the card , having adapters is not ideal ,

having intermittent problems such as this , do ( but not always the case ) point to possibley pick up of interference from say a motor starting or dirty contact etc

Also for VFDs a common mode choke (a giant ferrite bead basically)
around all three motor leads (U,V,W) can help a lot to reduce noise
induced by the square wave PWM from the VFD driving into the winding
to ground capacitance of the motor

Please be sure to keep us updated on your results. I am planning to run the same VFD + Mesa.

Marvin

I ended up buying a RS-485 PCI card. I also changed the cable to a CAT 6 cable. So far everything is working. I have not run the machine that much though. Some how in the process of working I ended up breaking my mesa 5i20 card. I think it had been going bad for awhile and finally died. There were times where the hostmot driver would not load and I would have to take the card out of the computer and reinstall it to get it to work. Luckily my plasma machine uses the same card so I can use that one until I get a new one. After I have to run an order of 300 parts so I can see if the problem is solved.

If reseating the card fixes things temporarily I would suspect the card/card slot of being contaminated. Cleaning the slot/card edge connector with 99% Isopropyl alcohol should help in this case.
Another possibility is a mechancal intermittant on the 5I20 card (possibly a bad solder joint) If cleaning the card/slot does not fix the problem the card should be returned for warranty repair.

I tried cleaning everything. I thought it might be the slot. but my other card works in both machines and this card does not work in either machine. I will send it in to see if it can be repaired.