Probably 3.00:1 rear end gears (that was pretty standard with the H-Code).
Start simple: Check your timing as Dan suggested... if the timing's not where it needs to be at lower rpms, it would have effects with higher rpms. If it returns to normal after shutting down, idling, and goes bad again at speed, that's going to be a tough timing issue to nail down. Maybe something mechanical sticking inside of the distributor or something.
Also check your fuel filter - if it's been awhile since a new filter (or if you had lots of problems with the rest of the upstream fuel lines since replacing it), you might have some gunk built-up inside it moving around that reached the magic point of restriction at the sustained rpms at 80 mph. If the blockage moved into a position to restrict the flow through the filter and into the fuel bowls, your carb would've starved until the bowl was refilled as the blockage fell away at the lower rpms (just a theory - seen it happen before, and fuel filters are cheap).
Consequently, if the carb's feeding too much fuel in at higher sustained rpms (flooding), that could be a problem that would need lower rpms to clear out as well. The float could be just that much too high, or maybe the needle valve is getting sticky when open or something to that effect. You would more than likely smell a LOT of gas at lower rpms if you were having a flooding condition, though.
I'd check the fuel filter before digging into the carb... and checking the things Jeff suggested as well before taking anything else apart. Like I said, always start simple.