"Make sure the printer only has the Power light on. Go to Chooser and select the 'StyleWriter II' driver and then click on the Printer port. Next click on the 'EPSON Stylus(EX)' and the Printer port should already be selected."
If this doesn't work, there is probably a conflict with another extensions (usually related to modem software).
Update: Florin Neumann offers the following insight as to what this problem is all about:
"The serial port used by the driver selected in Chooser should be set in PRAM. When a printer driver is opened, it gets the serial port choice from the PRAM; and when the port is changed, the change should be recorded in PRAM. Apparently this is what the Epson driver fails to do. When the driver is opened it reads from PRAM which port it should use, but when the user changes the port the Epson driver doesn't write the change to the PRAM, so the next time the driver is opened it will have reverted to the previously selected port."