Hi
Some Commodore 1200's had an missing capacitor, you would then see horizontale stripes on your screen every time your diskdrive is active. I don't know if you have this error since your screen flickers when you hd is active too.
I have alot of originale commodore service manuals but I can't seem to remember where I found this info. I just remember that the serial's of the affected 1200's were listed too.
Regards
HiZoap
