Strange overlay change - Parhelia 128

Hi there,

I know the spec for the Parhelia states that it supports overlays up to
1600x1200, but since I got mine I've been running it at 1920x1440 @ 81Hz
without any overlay problems (and even on occasion when I've put it up
to 2048x1536 @ 75Hz it's still run the overlay without problem). I have
it running in dual head mode with a 22" CRT monitor and a TV.

Anyway, I recently changed my motherboard (ASUS A7N8X Deluxe -> ASUS
P5P800), processor (AMD Athlon XP 2400+ -> Intel Pentium 4 3.4GHz HT)
and RAM (1 -> 2GB), and now the highest resolution/refresh rate
combination at which I can get overlay to behave correctly is 1920x1440
@ 75Hz. Anything higher than this results in a garbled, unmovable area
where the overlay should be, although DVDmax displays the overlay
material correctly. Not much use if I'm not using the TV, though!

Can anyone explain why this would happen? (And whether there's anything
I could do about it!) I realise it's not too bad to have to drop 6Hz,
but it's the principle of inconsistency that bothers me and I can't
understand why the change!

Thanks,

Paul
London, UK