CHARLESTON, W.Va. (AP) - A temporary halt in production on New Years Eve of a deadly chemical is among the precautions being taken by some leading manufacturers against toxic accidents and production foul-ups caused by Y2K computer trouble. Chemical manufacturers are confident that plants and production systems are safe from computer glitches that threaten operations Jan. 1. But brief production halts are planned as a fail-safe measure. Rhone-Poulenc, DuPont, Monsanto and Ashland Chemical all said they will idle some North American plant operations on the weekend when the calendar turns from 1999 to 2000. For the same reason, Rhone-Poulenc is also planning to eliminate its stockpiles of methyl isocyanate before the end of the year.