Wow, this is going to be hard to figure out. Like I said, many variables... You paid a lot of money for this, you should contact the seller if you haven't already. Since someone did the recap/mod for him, it could be a result from the work that person did, like a broken trace or a poorly soldered capacitor. It could also be leftover corrosion that the person didnt clean off of a pad. It could be wrong sync, it could be the low-pass filter or a lack of a low-pass filter. It could have too high of a voltage over the scart cable. Could be a bad connection on an RGB related component of the mod.
A million variables.