I thought the same thing, and since my batts were the originals and 10 years old, I put in new ones, but the problem didn't go away.
I also cleaned all of the grounds under the hood. No change. Changing the TPS made the issue 95% better though.

 

This is very strange issue, seems to only happen for me when its really hot out or I am pushing the truck hard.

 

That is how my issue started. Only happens when pulling heavy up a steep hill and working it hard. Then it started happening even unloaded cruising down the highway. A year after changing the TPS, it's happened twice.... Both while pulling heavy up a steep hill. It's like the computer thinks the truck should be moving faster and kicks out a P2121.

 

Yea I just changed the TPS a month ago, one from auto zone. I am thinking about just changing it again with a different brand, I have heard good things about the Timbo APPS so I might give it a try.

 

bump!