What do you have inside your Amiga?
Have you tried switching Power Supplies with your mate? A faulty 12v line on your PSU will affect video but not the computer.
Is your SCART cable a TRUE RGB cable or is it a composite SCART cable? (If it connects to the 23-pin connector then it IS RGB).
Try connecting the A1200 to an RGB monitor and to the TV via RF at the same time. You could also try the composite output.
Report back.
I personally think that your video DAC needs to be refluxed. 10 minute job if you're good at soldering.
