New DIY STC T12 Soldering Station - Works but got a small issue

[RayRay]

Hey guys.
I've built a new soldering station from parts (STC T12 based), and it works fine for the most part, but from some odd reason, the display temperature keeps constantly jumping up and down! (The temperature itself however is very stable and accurate, according to my tip thermometer), so it's just a display issue.  I've tried playing with the different settings (P00-P11) and made some research about what each of em does, but was unable to resolve it. If it helps, the main parts are these:
https://www.banggood.com/STC-T12-Digital-Soldering-Iron-Station-Temperature-Controller-DIY-Kits-for-HAKKO-T12-Soldering-Iron-p-1146578.html
https://www.banggood.com/4A-To-6A-24V-Switching-Power-Supply-Board-AC-DC-Power-Module-p-969204.html
I've followed the instructions fully, and double checked that everything is wired correctly.
I've also tried two different tips (just in case it was a faulty one) but that didn't help either.
But yeah, so to sum it up, almost everything works properly (except for the display temperature erratically jumping up and down)
Any ideas what the issue could be? I'm thinking it could be a faulty T12 controller, but any help would be appreciated.
I'm hoping I could get this resolved without replacing it, but we shall see.

[thm_w]

For some reason the thermocouple in the T12 tips takes time to "burn-in". So you will have to use it for maybe 1-2hr before it stabilizes.
Its possible the official hakko tips are either pre burned in, or use a different manufacturing process.

Seller explanation:

because with the new T12 tip, the tip inside the heater did not burnt, it will be just a little jump start with warm, generally do not need special treatment, the natural use of 1-2 days basically not bounce up. If you need effective immediately, the general recommendations of the temperature to 350 ?-380 ?while heating to the point of tin iron head, burning off after 4-5 minutes, until the tip to cool before you can reboot! (1 no action can be repeated 2-3 times)

Mentioned here also: http://dangerousprototypes.com/forum/viewtopic.php?f=56&t=5264&start=450

[RayRay]

I appreciate your help thm_w, but that's definitely not the case (and for the record, it's not true either, as I've seen plenty of youtube videos with T12 diy's and this was never an issue, the temperature display remained stable in all cases). My controller is definitely faulty.
It seems to be a grounding issue with it too. If I disconnect the grounding from the iron plug, the temperature display is stable, with it, it's erratic.
Guess I'll have no choice but ordering a new one.

[RayRay]

Update: figured it out.
Seems I've accidentally created a grounding loop.
I've previously used some files to slightly expand the encoder hole (as it didn't align proper with the display)
But in the process of doing this, I've removed some of the non-conductive paint in that area, which  subjected it to a grounding issue.
I've wrapped a small portion of the encoder with kapton tape, and it's all good now.