The parity error may occur consistently or intermittently. It is also normal to see parity errors in Windows but not in DOS because Windows exercises the memory much more than DOS does.
Parity errors are often a signal that something is working incorrectly in the system.
This is commonly a problem with the memory itself, but can be caused by many other sorts of hardware problems as well. You may be surprised at some of the hardware problems that can lead to memory corruption, including expansion card issues, resource conflicts, etc. This is why parity checking is so important. Look on the screen to see if the system is giving you any sort of memory address that indicates where the parity error is occurring. Reboot the system and see if the same address comes up again, and then reboot a third time.