Loss of power at cruising speed, like your hitting a head wind. But has power if you speed up.

check the cylinder compressions , could well be on the low side at that mileage

Could be the TPS. Especially if you have cruise control, use it a lot and tend to stay at the same speed all the time. If you do the TPS contact strip will be worn down at the point where you always set it at (i.e. say 70Mph) the metal contact at that point gets worn away since it sits on the same spot all the time. The rest of it is fine, but that spot is bad, when you hit it the power is temp lost, then returns once it goes past that part. Moral of the story. Don't always cruise at the same speed, your TPS will thank you!

check the rpm and road speed when this happens . The cruise control will increase to keep RPM constant but as the road speed drops the auto may be shifting down and as the rpm increase it shifts back before the rpm adjusts. You can override a cruise control in this situation by simply pressing down on the accelerator to maintain the power setting and road speed and when the situation is passed and you take your foot of the accelerator the cruise control will return the system to the set position without you needing to do anything

how old is the car?

could be several things
1-- coolant radiator is partially blocked or the thermostat is not fully opening reducing the flow of coolant through the radiator
2--- tuning is too lean at full load which causes overheat conditions for the engine
3--- timing a bit retarded for the rpms
i t is all connected to the extra load from the head winds requiring extra throttle that you would not normally use in normal conditions
another place to check is the fairing in use as the extra wind speed from the bike and the head wind may be causing a turbulence around the radiator and so preventing full air flow to the radiator

Generally... There is ON/OFF - SET - ACCELERATE/RESUME, COAST/DECELERATE. Cruise is available at speeds above 35mph or so. Sometimes there is no main on/off. Cruise on a turn signal stem has a button that pushes in on the end, push when at desired speed. resume/accelerate slides to right, is spring loaded and return left when released. the same lever when slid left is coast/decelerate. Momentarily sliding accelerate and releasing increases crusing speed by a couple mph. If brakes are used cruise disengages, sliding resume returns you to cruise speed after braking. Coast/decelerate disengages cruise, when at desired lower speed hit set. On some, hitting set not needed, when coast is released speed is maintained. Variations may be up/ down instead or may be steering wheel face mounted. Please rate answer. Duct tape is like the force, it has a dark side.. a light side.. and holds the universe together!

if i remember correctly ,, the computer drops a few cylinders while cruising to save fuel and this turns off when you step on the gas again ,, so there may be nothing wrong with your truck

Take it to a mechanic and ask for a coolant leak test, they attach a unit to the header tank which will detect anything that may indicate the head gasket is leaking. Should not cost more that a few dollars.
Cruise control issue is not related to the coolant loss, have them do a code check on the vehicle too.

ive had trouble with those before.its called a torque lock solonoid.It can be found behind the transmission pan on the drivers side of the transmission...cant miss it...big square pan on drivers side.

Sounds like the transmission went south on you..(surge and power loss was the trans not sending power to the wheels, allowing the engine to freewheel as the cruise applied more power to try and maintain speed).Depending upon how high the rpm's went when it let go, it may have damaged the engine, but that remains to be determined...Your mechanic will likely concentrate on that before putting a transmission on a wounded engine. The one grain of hope in what you said is that it remained running while you had the a/c on waiting for a tow. Good Luck!!