Hi Andy,
I tried your suggestions but nothing changed. Not on bot my macs with different browsers.
However suddenly something hits me.
It only occurs when a featured image is involved. When no featured image is set (and therefor no image is displayed on the archive page) the items behave perfect.
Could it have something to do with loading times of the images? As we live a mobile life we often have not a very fast connection to the internet. See screenshots. The first is the top of the page were images are used, the second one is near the bottom of the page and here are no images used.
I hope this input is useful.
Adri