Yep It's an MP distributor and because I've had problems with the MP curve, I sent it to Halifaxhops and he rebuilt and recurved the unit, but even then, what you describe is exactly correct as the vacuum and mechanical advance are both moving the timing significantly between 1200 and 600 rpms. If the trans pulls the engine down and it loses 5 degrees of advance and that means the idle drops even more...
Like I said I've done the usual due diligence with idle screw setting and searching for vacuum leaks. I think I'll even try another carb on it to see if it is internal to the carb.
Thanks to everyone for a speedy response - you've given me a lot to try. I'll let you know what happens.