Steering stiff, but headset not too tight...

Ok, kind of an on and off problem i have been having since i built my newest bike up a couple of months ago.
Have been through the process of preloading top-cap, then tightening stem, then finishing off top-cap. When holding bike off the ground steering is nice and smooth, wheel drops to the side nicely. However had some feeling the headset still had some play, however put this down to play in disc rotor instead as it didn't exist when both brakes were held or wheel was sideways.
So based on that, steering should be all fine, however there are times when it gets a bit twitchy and stiff, feels like it gets a little stuck when say moving back from 20-30 degrees off centre back towards centre. Symptomatic of a too tight headset, but the headset isn't too tight?
This was all fine for a while recently, however reappeared yesterday in the pissing rain, and has persisted today.

Any thoughts??
Do i need to grease the steerer post more? Could it be the crown race (i don't know, 99% sure it was properly seated when building)

Bike is a steel gravel/road frame and forks, with 33mm tires, disc brake wheels (mechanical).

Got a pic of the crown race? It could be that the headset is notched/indexed and that a new headset is required.

Will try and get one soon, maybe this evening if i have time.
How do you mean notched/indexed - would this happen out of the box, as this problem was there right from the off

i get a feeling that seems like headset play but is actually disc brake pads moving a bit. but this seems like its too tight?
is it a new steel frame? could the headtube be ovalised slightly?

Yes - agreed, put it down to disc brakes as it is only rocking when holding front brake only. Not when wheel at 90deg, or back brake on also. Will try to back it off a bit but not convinced this will be the cure, as the issue is a temperamental one - sometimes the steering is smooth as

Frame is brand new, so would doubt that

I had a similar issue and it was the fact that one of the headset cups was slightly at an angle to the frame. Pressing it properly solved it- but was a constant issue, not intermittent so yours might be something else. Worth checking though.

If it’s an older or recon bike, then it could be a bent steerer tube.

Cheers all so far. Bike is brand new (2 months) so doubt steerer tube is bent either, also problem would be constant I imagine.

Tonight it was fine, smooth steering all the way home. Go figure.

Will try and figure something out, half of me thinks it could be cables being slightly stiff/short

What type of headset does it use?

EC34/28.6 top cup, ZS44/30 lower cup
Think the parts I bought were nukeproof

Have been through the process of preloading top-cap, then tightening stem, then finishing off top-cap.

(my bold)

Out of interest, what do you mean by "finishing off top-cap"?

check wheel bearings
i had an issue with a headset stem and fork when building a new bike a few years back
after loads of faffing eventually worked out the wheel bearings were loose causing what felt like movement in the headset fork area

killed a thomson stem by trying to tighten it too tight to get rid of the play !

Just tightening a little further to secure it. I understand that since the stem is secured in place then adding a bit of load to the top cap afterwards shouldn't interfere with the headset tightness?
From memory i haven't overly tightened it, but i guess that could be a potential issue?

Sounds like either some bit of the headset isn’t assembled correctly or it’s a disc brake/wheel bearing issue manifesting as headset.

If there’s no play with wheel at 90 deg I’d say that most likely points to it not being the headset but it could be something misaligned in the headset. Was the headtube faced prior to headset installation?

it shouldn't affect the headset tightness but its also pointless, and your just adding extra load to the star nut, or fucking the threads on the bolt.
Some people remove this cap completely after tightening the stem to save weight!
once you've added the preload and tightened the stem bolts i'd just leave it well alone.

Interesting, will loosen that off again then, seems obvious now you think about it.
Will take a look at some of the other things mentioned here. Thanks all.

once you've added the preload and tightened the stem bolts i'd just leave it well alone.

This. No need to add extra load to the top cap after the stem is tightened.

Have you eliminated daft options like cables snagging?