[Mike Taylor 3601]

Use light w/o advance , advance timing lights will do weird stuff on msd when you rev,but I don't think they will actually change timing,just give wacky readings.
You said in post you could hear engine change sound like timing is moving,the only way light could do that is if it caused some kind of interference.

You have'nt changed anything on car that could created this?
If it has alternator be sure it's OK diode can go bad send (slight) AC current and make computer do weird stuff.
Check MSD box wires just to be sure in parts changing some thing has'nt got crossed,is battery full charged?I checked time on my car once batt.was a little low,starter was slightly draggy timing was jumping all over,was'nt that low but was enough to mess up stuff. Try wiggle and pull on wiring while it running listen for change you could have broke wire or connection that would show up.

I had pull truck worked on timing was going haywire mag pickup was bad,I know you changed dist. does your car use any kind of crank/cam/hall effect sensor or trigger.
Just kinda throwing out there any ideal that pops in my head.

Boy it be nice if you could run off ign seperate from computer to narrow it down
Mike Taylor 3601