This is just a guess, but if we were to assume the two things are related, I would be looking at the generator. GM went from the world's second best generator to the worst pile starting with 1987 models. Even when they keep the battery charged, their design makes them prone to developing voltage spikes that interfere with computers and the signals coming from sensors.
If you can catch it while the problem is occurring, use a cheap digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it stays around 12.0 to 12.6 volts, the generator is not working. You would have most likely had starting problems by now. Computer problems typically don't show up until the voltage drops to 10 or 11 volts.
If the voltage is within proper limits, unplug the connector on the side of the generator towards the rear of the case. If engine performance improves, suspect excessive voltage spikes as the cause. Before you jump on a new generator, we should look closely at the battery. It is very common to go through four to six of these generators in the life of the truck, but what more and more professionals are finding out is the repeat failures can be reduced by replacing the perfectly good battery with a new one. As the battery ages and lead flakes off the plates, its "internal resistance" goes up. Basically that means it still has the capacity to start the engine but it loses its ability to dampen those voltage spikes. The battery will work fine in an older car or truck without so many computers or that generator design, but it won't dampen the voltage spikes that are produced even by a new generator.
The voltage spikes come from the operation of the voltage regulator built into the generator. To reduce heat, it turns full on and full off 400 times per second. VCR power supplies work the same way. They are called "switch mode" power supplies. Since the current flow through a coil of wire is being switched off instantly, it naturally develops a big voltage spike. That is how an ignition coil develops a spike that can be transformed into thousands of volts to fire a spark plug, but those spikes aren't so welcome when they transfer magnetically into sensor wires. The funny thing is the '86 and older GM generators and the Chrysler systems work the same way but they don't have voltage spike problems. This is strictly an '87 and newer GM thing.
I don't know if I'm leading you down the wrong road, but it is such a common, yet hard-to-diagnose problem, that it was the first thing that came to mind. One potential way to find it, besides the simple unplugging it test, is to drive it with a scanner connected that has record / playback capabilities. When the problem occurs, you press the record button. Since the live sensor data travels through the scanner's memory before it is displayed on the screen, the recording actually starts a few seconds before the button is pressed. That makes finding intermittent hiccups a lot easier. Later, when the recording is played back slowly, each sensor can be viewed to look for a glitch, spike, or dropout when the problem acted up. If a couple of sensors are affected at the same time, that would point to a generator problem.
Monday, July 5th, 2010 AT 3:16 PM