driver devg-stpc

Driver devg-stpc doesn’t work in 1024x768x16 resolution.
With devg-svga and devg-vesabios I have no problem, it’s work fine. But with
devg-stpc 1024x788x16 I have a totally black screen.
(smaller resolutions work fine even with 1024x768x8)
Any idea or solution ?
thanks.