Crappy converters can be the cause of many problems. Though, yeah, it could be a sync issue too.
I think this is the converter people have generally recommended in the past:
(see here, for example: Composite to RGB conversion? )
Seems to be out of stock at the moment, both via Ambery there and on Amazon.
I don’t have one, so I can’t comment on its abilities. For the price, though, I wonder if you might be able to find a different Extron converter (used, presumably) that will take composite input?