my friends truck when you go over 50 mph on a hill its like some one turns the gas off then turns it back on real fast . this only happins at 50mph and only on a hill can some one figure this out?

 

Like a misfire or is the truck actually stalling out

 

I would first change fuel filter, $10, just as general maintenance, and it could be 15 years old.

EFI Fuel pumps are not on all the time, computer turns it on and off based on RPM, speed and load.
Could be dirty fuel filter is limiting flow when it is needed, under load(uphill) and higher RPMs, and computer is turning on the fuel pump just after the stall(normal routine at 50mph), so flow comes back enough.


The Throttle position sensor(TPS) tells the computer where your foot is on the gas pedal.
The inside of the TPS is like a light dimmer, or volume control, so a simple variable resistor.
These can get "dead spots", although it is not common.
They are very simple to test.
They have 3 wires, 5volts from computer, Ground from harness, and the Voltage return wire that goes back to the computer.
So the computer sends TPS 5volts, if foot is off the gas(idle), TPS sends computer 1volt(usually .9v).
If foot is to the floor(WOT), TPS sends computer back 5volts(usually 4.8v)
The in between voltage is what you need to test.
Get a Volt meter, set it for 20vDC
With key on engine off, use a couple of sewing pins to pierce the Ground Wire(or use any handy ground) and Return wire(the one that is not 5 volts)
You should be reading .8/.9volts.
Now slowly open throttle, voltage should increase as you open throttle, nice and steady, no jumps up or down in voltage.
Go to WOT and then repeat a few time, a "dead spot" can easily be seen as voltage jumping or dropping.
Never replace a sensor, always test them, sensors rarely fail.

 

the truck at 50mph and up will buck then be ok on flat road then you hit a hill doin 65 or 70 then it starts bucking. we changed fuel filter

 

Clean the MAF sensor, it may not be reporting correct air flow at higher speeds.

What it reads like that at 75%(estimate) throttle everything is fine, you hit a hill and to maintain speed you open throttle a bit more, so at 80% things are not fine, could be TPS sensor or MAF sensor