Never heard of that problem before, it should not do that. Some of these trucks are configured with a speed sensor that splits off and goes to the ECM, and the other goes to the speedo head.
It is possible the speed sensor could be falling out, but a way to prove that is to watch your vehicle speed on Nexiq to see if it also falls off. If it does not fall off in the ecm, but falls off on the speedo, there should be 2 connectors coming off of the speed sensor, you can swap those to see if you lose vehicle speed in the engine ecm, but hold vehicle speed on the speedo head.
When I use Detroit Diesel for a cylinder cut out, you look at the baseline pulse width, and if any injector goes below the baseline, it could be 1 of two things, air in the fuel, low fuel pressure, faulty injector. None of these symptoms could cause a fault code. The only way a fault code will show in the ecm is if the response time of the injector is long or short. This is typically a injector solenoid, or harness issue.
As far as fuel pressure, depending on where you put your gauge, Detroit recommends you put it where the fuel temp sensor is.
At idle, 600 rpm, you should have 15-22 psi
1200-1300, you should have 30-45 psi
1800-2100, you should have 65-80 psi
If the fuel pressure is high, I would suspect a plugged fuel filters or faulty fuel pressure regulator. If the guage is really bouncy, and unstable, especially under boost, then compression could be leaking past the injector o-rings. Or air is getting in the fuel somehow. Which you eliminated the suction line by going straight to the fuel tank.