On Amiga related forums there are supplier who sell these DIY adapters. Commodore made these adapters back in the days but they are very hard to come by. I’ve seen adapters in eBay also, but one must take care to actually order *BUFFERED* adapter, not just straight wire adapter.
Like said, this is no fault in the OSSC. Amiga users have the same problem with VGA monitors. If monitor happens to load sync signals too much, Amiga assumes it should lock to external sync and things go downhill.