I know you've tried adjusting the trimpot, but it does still sound like the ill-tuned deinterlacer.
Did you try adjusting the trimpot when the computer is switched on? This will allow you to see whether tuning is having any effect at all.
Will connecting to the RGB port give you non-scandoubled output? Does Amber affect the 23-pin output? If not, then it is not surprising that you get the rolling bars from that port.
Regards
Rich
