my truck deacclerate at 40 to 45 mph. just for a sec.
Is the check engine light on? If so, have the codes read and post back with the results.
Your fuel pump could be dying. That can be checked by measuring the fuel pressure.
If it has a TPS Throttle Position Sensor, it may have a small dead spot in its range of resistance. Electronic parts cleaner might fix that.
From what I can find it doesn’t. And wouldn’t the fuel pump act up at other speeds?
You’d think so. The only thing that is totally speed related that could do this is the VSS Vehicle Speed Sensor, and whatever its signal goes to. Trans? ESC? Ignition timing? Fuel system?
It would help if we could be sure that some factor other than speed is at play here. Could it be a shifting problem?
At this point it could be almost anything. I’m out of ideas and open to suggestions.