What is this part?

It's on the steering column, next to firewall. Looks like a reluctor /position sensor. Has "5136 GM/A" on it. Can't find it in any parts catalog.
Thanks

Let's see a pic.

Photo attached

That is the steering wheel position sensor for the variable rate power assist.

This transducer shows the "slew rate" of the steering wheel for the Variable Effort Steering. If the steering wheel is being moved at a high rate, as in an impact avoidance maneuver, the VES goes to full boost.

Thanks guys, reason I'm asking is because my power assist intermittently cuts out entirely.

Try unplugging it and see what happens.  It'll default to full boost.  If the steering is still erratic, you likely have a slipping serpentine belt, in which case the solution is to replace the serpentine belt and tensioner.

EDIT -here's a writeup I did when I took one apart and cleaned it out. Look for post #43.

And... don't forget to make sure your water pump weep hole isn't leaking. Feel underneath. When cold. The leaking water pump causes your symptoms on cold starts, at low speeds, under high effort like parking.