I am having exactly the same behavior, and it's even worse as it approximate the meridian.
My guiding is very good giving me an error between 0.3 and 0.5 but stars get oblong above 60 degrees altitude.
One thing that I am going to check is the counterweight, instead of having only one at the end of the shaft, I will add another one and distribute the weight on the shaft.
My balance is not perfect too, I have it more heavy on EAST side.

Unless my box has been nudged, my alignment was spot on indilib.org/forum/general/3504-finally-i...-6-months.html#26793

Try Binning 2x2. Box size 64 is far too large, what are you guiding on? a Quasar?

On EQ8, I guide for hours < 1" so it's unlikely to be a software issue. Try with PHD2 as well, it is far better than the simple proportional guiding done in Ekos. If someone wants to import PHD2 to Ekos, be my guest

This could also be related to the issue Herrausen has in another thread about the internal guider.

-Eric

Just looking for more detail... why is a "Box Size 64 far too large?"

Correct me if I'm wrong: if you are guiding on a "normal" star, the mass should probably not exceed 8x8 pixels. Because the objective of guiding is to maintain a sub-pixel centroid position, 64x64 is too much unused frame data in a real-time system where the correction needs to be close to the sampling instant. Now, that opinion can be discussed say in the case of star loss because of passing clouds, and of guiding robustness more generally.

-Eric

I have a QHY5, binning is only 1x1 I'm afraid.
I did try 64 as the star I selected was quite big.

Looks like wind to me

By the way, maybe you have the same problem I had. My mount was oscillating when I was high in the sky. Decided it was the mount and I rebuilt it with a belt drive modification. Don't tell me it was Kstars! lol! Check out my video:

Almost the same issue here. Funny though, because BEFORE the meridian Flip guiding was working quite well, but after the flip totally out of controll.
Could this be weighting-issues?

You could try to recalibrate after the flip to see if it has an impact with your setup.

-Eric

Good point, i remeber guiding started immideatly after flip without calibrating. I´ll have a try tonight.

There was no wind that night at all.
Nice video, I heard so many good things about the belt mod. I'm going to get it now and install it in December.

When I slew the mount, it sounds like a bag of nuts.