I was in a similar position as yourself. Although i had a sort of working MBFM, i couldnt get my finger on the problem (I did have sound however it was faint and with lag).
I decided todo a complete rebuild of the OPL3 module. I ordered a new pcb from Mike and ordered all the parts myself. Only reused the IC sockets and the TL074.
Two weeks ago i soldered the whole thing together and to my surprise it was actually working. If u need it, i may have a spare YMF262 for you. It is a pull though, not a new one.