How does it look on the plasma display? It is likely simply artifacts of the scaling and video processing in the scaler and/or projector, that will show up most noticeably on things like sharp edged objects with lots of contrast that are moving, which is exactly what the scrolling text is.
I can't remember if the SF-251A is one of the scalers that locks the output to the PC input settings or if it can have the output set independent of the inputs, but in either case you really want both the PC and scaler set to match the native resolution of your projector. If not, you end up scaling twice, once at the scaler and once in the projector.
Since it has not been mentioned elsewhere, all LCD and DLP projectors have internal scalers, that is how they can display video images or remap other computer resolutions to their native display resolution. However, they then need to rescale and resync every time you switch inputs, leading to delays and screen artifacts. The purpose of a scaler/switcher or seamless switcher is to move this switching and scaling outboard so that you can potentially have a higher quality scaler and can always send the projector it's native resolution so that it never has to switch or resync. A true seamless switcher uses two internal scalers so that the rescaling of both the current and next selected source is accomplished prior to switching.
