Yahoo show different results every time is because Yahoo fetch result from one of the datacenters of Google and process it a bit and then post it too you. Each access to the datacenter is random. So I bet your listing is showing in some datacenters and not others, so Yahoo might show and not showing your site depends on which google datacenter it access.
AthlonInside, as already explained in msg#1, this problem is not related to a data centre. Each data centre is showing the same problem.
Also, the access to different data centres doesn't explain the behaviour which I saw today (see msg#23). Also, the appearance/disappearance with a probability of 50% (see msg#23) would only be explained by the access to different data centres if the data centres show different results. However, most of the time I saw the same results on the different centres, but changing (on each data centre) when reloading the page.
I'm totally happy to check out individual examples if people think there are problems, but I need data. :)
I'll fill out the spam report. I would be happy to hear any kind of response.