Car dies in gear...

So this is strange, (or maybe not?)

I put a quart of ATF in the Trans and now it works....

So no change to the carb or anything besides adding fluid.
Idle rpm 800, drive RPM 550.

Idle vaccum in nutral 20", idle vaccum in drive with the brake on, 17".

I'm waiting for the car to warm up and the Trans to warm up, then I'm gonna take the car off the stands, check the Trans fluid and MAYBE go for a test drive! (If its still working once warm)

Could it have been as simple as putting fluid in?