Well, cyb is saying he had an issue like this with a PVM-20L5 and was able to fix it using an extron unit. And Calamity was saying it looked like a front porch issue, which is sync-related as well. I'm not sure if this will work for you or not, but if you don't want to try recombining sync for yourself with a breadboard, some breakouts and a pack of resistors, then getting an extron isn't a bad idea. Extron make many different devices, and used to make tons more, and many of them are really good at combining sync, doing so very cleanly.
Yes, it would be PC ---> VGA cable ---> Extron ----> 4 BNC cables for RGBS (or RCA+adapters) ---> PVM-20L5
The 20L5 will take 480p via RGBS happily.
I meant not scaling as in not upscaling or downscaling, keeping the same resolution and timings. So with the extron 190F you feed 480p in, and 480p comes out, or 720p for 720p, etc. ignoring any signal formats here. Obviously you're converting RGBHV to RGBS in this case too, and there is a function for horizontally shifting the picture, so remember to try this out as it might help fix things too. The extron 201 and 203, IRC, are taking whatever you feed in and turning it into 240p/480i. Or they can do that but don't have to? Either way they cost a bit more and you might be paying for something you don't need. That said, take a look at those units and see if they're doing something you might want. It's worth looking up the documentation for any extron you're thinking of buying to check it out, most of them would be pretty cool to have in general.
If you do grab one, report back with results please