This sounds like fuel flow maybe. Can you rev the engine to near redline in neutral without a problem - opening the throttle to about 3/4 and it should quickly spin up without any bogging (except off idle which is normal unless your bike has the accelerator pump) which says at least some stuff is working fine. Riding at 60 takes a fair bit of gasoline flowthough. If the petcock or fuel screen is clogged and only a trickle is flowing, the reservoirs in the carbs will fill and supply gas for revving or accelerating through the gears but constant demand at cruising speed may drain the float bowls and the bike dies.
Remove the petcock fuel line(s) and run some aquarium hose from the petcock into a jar. Turn on the fuel and see how much flow you get... I don't have any specs but a cup of gas should appear in less than 30 seconds with a full tank. If there's a problem, the petcock is easy to remove once the tank is empty - either a big nut above the petcock or 2 screws from inside the tank - but some gas WILL spill out anyway, take the tank off and do it outside! The mesh fuel filter (usually comes off with the petcock) can get gummed up and clogged, and the inside passages in the petcock itself can get all clogged with crud. If the petcock has screws holding on the escutcheon plate around the tap lever then it is easily disassembled and cleaned, riveted ones are less friendly but you can drill out the rivets and put it back together with tiny screws if you're handy with that kind of work. The mesh screen generally disintegrates if you try to clean it but it's worth a try; you can safely do without it if you install inline fuel filters in the hoses to the carbs. You should install those filters anyway as they prevent most float valve leaks and carb drips - dirt particles small enough to pass the screen can and do lodge in the float valves and make them leaky.
The other usual issue is ignition. Even if the bike revs freely to redline, cruising throttle can give cylinder pressures that a weak ignition can't ignite. If new spark plugs solves your problem, excellent. You may need new spark plug caps too, the original plugs were non-resistor and the caps had resostors... non-resistor plugs are now near impossible to find and the double resistance can weaken the spark. Either get non-resistor plugs or non-resistor caps... one can be resistor but both has caused trouble for me.
Last is carburetion. Set the points gap and ignition timing as close to spec as possible and check that the spark advancer is working properly.
Once you know you have fuel flow and your ignition is set to spec and your plugs, caps, and wires are all good check the fuel/air mixture. Run the bike for about a minute on the highway in the "bogging" condition then switch off the kill switch, pull the clutch in, and coast to a stop (shift down as you slow so you're not stopped in top gear!). Check the plug tips colours immediately and if they are not a nice grey then your carbs probably need work. Oily plugs anytime means you have oil leak trouble, piston rings or valvestem seals most likely.
