Has this issue been discussed? Seems common online. 07-09 impala with the 3.5 suffers from a reduced power, service traction control issue which may be due to tps sensor and/or gas pedal sensor? You can replace the tps or the throttle body to fix the under the hood issue.
09 Impalas had the warranty extended on accelerator pedal sensors, but the vehicle had to have the condition (as you possibly describe) at the time they go to the dealer.
I seem to recall that is a redundant (dual sensors) drive-by-wire) pedal set-up
Don’t try this at home: 
Some years back, wife’s car had the problem and I got the symptoms you describe and a code for it.
I unbolted the pedal assembly, unplugged the harness (a bit of a puzzle with safety clips), put some dielectric grease on it and plugged it back in. Voila! Fixed!
That was years ago, tens of thousands of miles, and it has never had problems, again!
Fretting corrosion, as GM calls it? That’s my guess.
GM can’t do that for liability reasons. If they act up they replace them. period.
This may or may not be your problem. Check the code and you can probably try what I did.
CSA
P.S. I can’t remember if I was able to clear the code immediately. Some codes for critical components take many drive-cycles to clear, but the car worked properly immediately.
This is just a side point, not related to the primary question. @common_sense_answer, you said you used dielectric grease. I’ve used that, too, on difficult electric connectors. I finally looked up “dielectric” and it means “not conductive”. So, it’s meant to be used to lubricate where insulators meet like spark plug caps and the plastic housings on electric connectors, but not on the actual contacts. I was using it all wrong all these years.
I visually inspected the actual electrical pins and sockets and was expecting to see corrosion. I couldn’t see any, but sometimes it’s nearly invisible to the naked eye. I believe that’s what GM refers to in bulletins when they call it fretting corrosion.
Just unplugging and replugging these connectors usually fixes it, but to protect it for the future I us the DE grease.
I use it on the bases of light bulbs, too.
CSA
I’ve seen electronic techs apply a dab of dielectric grease to connector pins directly. It presumably gets wiped off, pushed out of the way of where the spring loaded pins make electric contact with their mates, as the connector is mated to the other half, but still leaves a protective moisture repelling layer surrounding the pins, which might help to prevent oxidation of the connector surfaces. I think DE grease has a high breakdown voltage; i.e prevents sparks from jumping through it. So its used in the high voltage section of the ignition system too. For the spark plug boots, the greasy property makes removing the boot easier, a side benefit.
I just assumed it helped make better connections, until I found out that’s not the point at all. It was one of those moments.
Dielectric grease is NOT to be placed on the actual contacts. Even a small film can interfere with electrical signals.
I understand and appreciate your concern. However, that topic is open to great debate. Google it and you’ll see. At least one electronics guy carefully tested connections with and without and the results were normal with DE grease. I come down on the side that says to apply it to electrical contact points.
I have been doing this for years and years and never a problem. However, it has solved fretting corrosion problems. My proof is in my pudding.
For those a bit squeamish about using it this way there are companies that make a grease that is not dielectric, but keeps out moisture and corrosion and actually promotes healthy conductivity.
Back when we had a Radio Shack nearby I liked keeping a spray can of their “tuner cleaner” on hand. It cleaned and left a protective barrier, too.
CSA
From my experience with connectors, although not in the automotive world:
Most connections are tin on tin, and tin quickly develops a coating of oxide which is a non-conductor. The connection depends on the wiping action when the connectors are mated, to wipe off the oxide. I’ve never used dielectric grease but I can see it’s value in providing a seal to prevent further oxidation of the tin. (see below)
The alternative is gold on gold, where there is no oxide layer, but that is expensive.
Also, many crimp connectors, such as ribbon connectors rely on a gas tight seal to connect the wires to the pins, so that once the oxide is wiped off, the connection is sealed and air cannot get in to form a new oxide layer.
edit: but I think that for low level signal connections, thee is a possibility of a thin layer of the dielectric grease causing a bad connection.