I've been wrestling with an SDRAM problem with the newer Megadrive core on V2 hardware which I believe is now solved - so here's an updated core, which brings some very nice upstream improvements to the Chameleon (both V1 and V2)
https://www.patreon.com/posts/megadrive-beta-51182147
http://retroramblings.net/?page_id=1604
Enjoy!