I had this happen about 10 years ago one day while I was driving my '69 RS to a weekend cruise in at a town about 50 miles away. When I started the car up at the house, everything was fine, good idle, etc. I left for the cruise and when I exited the interstate and came to a stop at the end of the exit ramp the car was idling very rough and died. It started right back up but idled badly.
I checked all of the vacuum lines as well, found nothing, then noticed a gap between the base of the carb and the intake manifold where there no longer was any gasket material. To this day, I never did figure out why the gasket failed - but there was my vacuum leak. Pulled the carb, installed a new gasket, and all was well.
Definitely sounds like vacuum leak somewhere. If it isn't the obvious (hoses) then you have to start looking for where a vacuum leak can occur, may not be a likelly candidate, but after eliminating the hoses .........
The car runs fine going down the road because the extra air being drawn into the engine via the vacuum leak isn't enough to lean out the air/fuel mixture while you've got your foot in the throttle. At idle - too much air.
