I would tend to agree with "Strat", that you might inspect the quality of the wiring, especially if it's over 10 yrs. old in a "shop" environment. It's possible that coolant, mist, etc. have broken down the insulation over the years and this could impact signal integrity. In addition I would suggest you check the routing of the cables with respect to the proximity to high voltage sources, motor leads, etc.. I have seen voltage transients cause some pretty bizarre issues with RS-232 communications. Finally, insure that the shield of the cable is only connected to pin 1 (assuming DB-25 connector) on ONE END of the cable, otherwise the possibility of ground loops is almost unavoidable.

All that being said, the reliability of RS-232 based communications in a factory environment is pretty good, provided that the above pitfalls are avoided. I have heard many people advise against running over 75 ft., but the driver IC's that are used in current equipment can usually handle a couple hundred feet. Of course, USB to RS-232 has it's own issues, but based upon the vintage of the equipment you seem to have, that is likely not a consideration.

As someone else mentioned, Suburban Machinery is still around (Suburban Machinery / Software Division Home Page) and Dan Fritz is a very helpful individual.