Changing from 8x to 4x AGP on Abit PT880

I have just changed to the above motherboard with the VT 8237 chipset. I
also have an nvidia 6600GT AGP gfx card. The problem is the card is
hanging on 3d apps and in the past I have had to turn the AGP down from
8x to 4x to stabilise things. im not too bothered about doing this as
the performance hit is almost unnoticeable.

My question is, does anyone know how to do this with this motherboard ?
I go to advanced chipset features where it shows the AGP speed but it is
greyed out and cannot be changed.

Any ideas ? any help much appreciated

Si