I have been getting these lines where there seems to be not enough filament extruded accompanied by some clocking noises (not really grinding). Recently I got a few errors about stuck filament as well.

I did a few cold pulls but there wasn't any debris to be seen and the three ends (cht) were clearly visible.

Noises, pattern and errors persist.

Pla, core one, max layer with 0.4 nozzle.

Any help appreciated

Edit is the image not loading for everyone?

Without knowing what printer you have I offer this advice:

Does your printer keep the temperature? I had a creality cr6 that underextruded and in my case it was bad capacitors on the heating element circuit.

It is a core one and the temp does seem stable

Nozzle might also just be too close to the bed.
Mine did a kind of of "tock" sound when it struggled to extrude.

PS: image link seems broken

If temp is stable try having a look at the filament. Is the extruder grinding the filament?

It could also be a case of a blocked nozzle. Though that doesn't explain the stuck filament. scratch that I just reread your post

When starting with an unpowered printer, i.e. the first print after powering printer up, does the issue happen at any part of the print, or only after a little while?

That (or something like it) happens to me with some bad filament. It gets stuck in a blob inside the printer head between the hot end and the gears pulling in the filament.

I have to take apart the printer head and pull the hardened blob out. I actually have to do it now.

Picture works now. It starts immediately

And no grinding on the filament.

Edit thr indx upgrade is coming soon so a thorough rebuild is coming too.

Since I have the end of the pulled filament and can see the 3 times split from the cht the blob is either not there OE inside one or more of the smaller channels after the split.

That should atop after a few layers though, right?

How about your filament path? Is there high resistance? The filament roll, is it hard to roll on your filament holder? How about reverse bowdens?

Is the extruder gear too tight or too loose?

Maybe a rebuild will fix whatever is causing this. Because when you experience:

1. Stuck filament
2. Underextrusion

That does sound like a strictly a failure in the filament movement system. And I can only think of

1. Grinding extruder, which can also be a symptom of the following
2. Unstable temperature
3. Clogged nozzle

And since clearing the nozzle didn't work, and you don't find signs of grinding, nor observe temperature swings, then I'm out of ideas.

Do you happen to have spare nozzles? Maybe try swapping the nozzle and see if it was still clogged? I've had nozzles that I just couldn't clear.

Not necessarily since the initial layers don't have the space to be squished properly, subsequent layers can show the same problem, except they're too close to the previous layer instead of the bed itself.

That said, the picture didn't load before but now it does.
With the picture, this looks kinda like wet filament to me where it'll kinda bubble here and there.
I've also spent way too much time hunting down underextrusion what was ultimately just the filament that had a weird undersized diameter like 1.6mm.
Diameter that's off but consistent through the roll can be compensated for in the slicer, inconsistent diameter not so much.

I found that new rolls seem to not want to unroll as easy because of the weight. If I have this problem, it's usually on a new roll.

A low friction filament path solves a lot of under extrusion issues. See if you can build a filament holder with ball bearings and/or a filament guide before the extruder. It solved all my issues with my former Ender 3.

Had something like this happen on my old ender 3. I replaced/upgraded the hotend and it went away

If this is a core one you should be asking Prusa, not because we can't or don't want to help, but because that's part of the price of the printer. And they will have a much better idea of what is wrong.

Good point.