2008 Chevrolet Impala LT Reduced Power Mode issue

A relative of mine is driving a 2008 Chevrolet Impala LT with the 3.5 in it.. if you can believe it, they have it at over 255,000 miles! Something of a miracle for Chevrolets, I’m told.. but then, they always did take good care of their cars.

Here’s the issue:
The car runs fine and accelerates fine… until they exceed 70 miles an hour. Now, they drive long distances and on some of the highways in Ohio, he can be doing 70 and have his doors blown off by other vehicles going much, much faster.
When he exceeds 70 miles an hour:
1) The car shudders like the engine is chugging.
2) The tachometer cuts out completely, reads 0.
3) The engine announces Reduced Power Mode and the CEL comes on.
4) The car will usually restart immediately after being turned off. Occasionally, the car will take a minute before it’ll run again.
5) Once restarted, it runs fine until he exceeds 70 again.

Some of the CEL codes that have been announced:
Throttle position sensor (Sensor was replaced)
Throttle PEDAL position sensor (assembly was replaced)
Crankshaft position sensor.

Other issues addressed:
Mechanic tested the alternator, said it tested fine.
The car has had the old battery replaced. The new one has been replaced twice due to it ‘testing bad’.
Purportedly when brought in for maintenance, the mechanic showed him the dropped transmission pan and there were chunks of gear teeth in the oil. We’re not sure if that was a scam attempt or not, because the car has been running for a while.

Again, the car runs just fine and doesn’t appear to have issues until they exceed 70mph. If the car is at 70mph and on cruise control and it goes up a hill, then the situation manifests again.

Am I wrong in thinking this is leaning towards one of the sensors being outta whack?

What do all y’all think?

-Hinoki