At a steady speed of 55mph, my truck will sometimes suddenly loose some power and drop to around 50mph. Then around 30 seconds later, the truck will suddenly gain power back and get up to 60mph. I figured somethings wrong with the turbo or the injectors are bad. Or, could it possibly be the overdrive on the 5spd (basically 4spd because of the 1st bulldog)?

 

R u sure its not a break hanging up does it pull to one side when u stop?

 

have you done any mods to your truck if so list the mods that been done to truck there few things i seen do this

do you have any codes going on

crack position sensor at them speeds
boost could also be problem
also seen Optical Sensor filter harness do that
had PMD to also

IAT is intake air temp and it has been written that speeds around 75mph and above can cause the computer to limit the fuel to the engine because the boost creates too much heat in the intake air which results in less fuel being injected and a lack of power.
I have not experienced this problem yet so I don't know if it is like it shuts down or just gets lazy? i know your are lower but wouldn't rule out

 

The parking brake line is snapped so I don't think it's a brake problem. It's also a mechanical injection. And it's all stock.