HOLY F*ING MOLY!  Hit it out of the park on the first shot!!

It's my goddamn SCART cable!   AHHHHHHHH!  the madness!    :D

VBL_INTR.PRG plays correctly when connected to a trusty old C1084S.  When I plug in the SCART setup, I suddenly get the wacky music.  And it changes on the fly if you plug or unplug it.  It's like some electronics carnival trick from hell.   :o

Actually the SCART cable by itself doesn't cause it, I have to connect the cable to the SCART switchbox, or directly to the component converter for it to cause it.

Of course this is a different problem as I need the SCART setup to work with my modern display, but at least it is quantifiable now.

Thanks for all your help guys, you are the best!  :)