OK, making progress, I assumed by the 12" of vacuum that you had a big cam in this motor. Now you say that when it's acting up you have a slight lope to it, now by this statement I'm assuming your motor is stock (or close to stock). So, vacuum should be much higher, more like 16-18", and with the carb whistle you probably have a vacuum leak. Check the vacuum plug caps at the the carb make sure they are all in place and good shape, and check the PCV & transmission hoses for cracks or splits.
Curious is the trip up hill one direction?