If the "Parallel Mode" option is programmed, and then the PROM is reprogrammed without the option, the parallel bit is not reset. This causes the PROM to continue programming in parallel mode.
In order to reset the parallel bit, you must first to a complete standalone erase. Then you can program the PROM without checking the 'parallel mode' option and the PROM will then program in serial mode.
This problem will be fixed in the next major software release.