With presently available modchips it is not possible to boot from PAL originals on an NTSC PS2 (or vice-versa). You have to make a backup as per the outline I provided.

I must be honest and say that in all the time I've been faffing around with the PS2 I haven't tried booting normally for swapping and then putting in a PS2 original rather than a backup. Is that what you've done? From your note I assume you've tried booting from the PAL original.